In most full-term infant boys with cryptorchidism but no other genital abnormalities, a cause cannot be found, making this a common, sporadic, unexplained (idiopathic) birth defect. A combination of genetics, maternal health, and other environmental factors may disrupt the hormones and physical changes that influence the development of the testicles.

- Severely premature infants can be born before descent of testes. Low birth weight is also a known factor.

- A contributing role of environmental chemicals called endocrine disruptors that interfere with normal fetal hormone balance has been proposed. The Mayo Clinic lists "parents' exposure to some pesticides" as a known risk factor.

- Diabetes and obesity in the mother.

- Risk factors may include exposure to regular alcohol consumption during pregnancy (5 or more drinks per week, associated with a 3x increase in cryptorchidism, when compared to non-drinking mothers. Cigarette smoking is also a known risk factor.

- Family history of undescended testicle or other problems of genital development.

- Cryptorchidism occurs at a much higher rate in a large number of congenital malformation syndromes. Among the more common are Down syndrome Prader–Willi syndrome, and Noonan syndrome.

- In vitro fertilization, use of cosmetics by the mother, and preeclampsia have also been recognized as risk factors for development of cryptorchidism.

In 2008 a study was published that investigated the possible relationship between cryptorchidism and prenatal exposure to a chemical called phthalate (DEHP) which is used in the manufacture of plastics. The researchers found a significant association between higher levels of DEHP metabolites in the pregnant mothers and several sex-related changes, including incomplete descent of the testes in their sons. According to the lead author of the study, a national survey found that 25% of U.S. women had phthalate levels similar to the levels that were found to be associated with sexual abnormalities.

A 2010 study published in the European medical journal "Human Reproduction" examined the prevalence of congenital cryptorchidism among offspring whose mothers had taken mild analgesics, primarily over-the-counter pain medications including ibuprofen (e.g. Advil) and paracetamol (acetaminophen). Combining the results from a survey of pregnant women prior to their due date in correlation with the health of their children and an "ex vivo" rat model, the study found that pregnant women who had been exposed to mild analgesics had a higher prevalence of baby boys born with congenital cryptorchidism.

New insight into the testicular descent mechanism has been hypothesized by the concept of a male programming window (MPW) derived from animal studies. According to this concept, testicular descent status is "set" during the period from 8 to 14 weeks of gestation in humans. Undescended testis is a result of disruption in androgen levels only during this programming window.

Most cases of polyorchidism are asymptomatic, and are discovered incidentally, in the course of treating another condition. In the majority of cases, the supernumerary testicle is found in the scrotum.

However, polyorchidism can occur in conjunction with cryptorchidism, where the supernumerary testicle is undescended or found elsewhere in the body. These cases are associated with a significant increase in the incidence of testicular cancer: 0.004% for the general population vs 5.7% for a supernumerary testicle not found in the scrotum.

Polyorchidism can also occur in conjunction with infertility, inguinal hernia, testicular torsion, epididymitis, hydrocele testis and varicocele. However, it is not clear whether polyorchidism causes or aggravates these conditions, or whether the existence of these conditions leads sufferers to seek medical attention and thus become diagnosed with a previously undetected supernumerary testicle.

One of the strongest arguments for early orchiopexy is reducing the risk of testicular cancer. About 1 in 500 men born with one or both testes undescended develops testicular cancer, roughly a 4 to 40 fold increased risk. The peak incidence occurs in the 3rd and 4th decades of life. The risk is higher for intra-abdominal testes and somewhat lower for inguinal testes, but even the "normally descended" testis of a man whose other testis was undescended has about a 20% higher cancer risk than those of other men.

The most common type of testicular cancer occurring in undescended testes is seminoma. It is usually treatable if caught early, so urologists often recommend that boys who had orchiopexy as infants be taught testicular self-examination, to recognize testicular masses and seek early medical care for them. Cancer developing in an intra-abdominal testis would be unlikely to be recognized before considerable growth and spread, and one of the advantages of orchiopexy is that a mass developing in a scrotal testis is far easier to recognize than an intra-abdominal mass.

It was originally felt that orchidopexy resulted in easier detection of testis cancer but did not lower the risk of actually developing cancer. However, recent data has resulted in a paradigm shift. The New England Journal of Medicine published in 2007 that orchidopexy performed before puberty resulted in a significantly reduced risk of testicular cancer than if done after puberty.

The risk of malignancy in the undescended testis is 4 to 10 times higher than that in the general population and is approximately 1 in 80 with a unilateral undescended testis and 1 in 40 to 1 in 50 for bilateral undescended testes. The peak age for this tumor is 15–45 yr. The most common tumor developing in an undescended testis is a seminoma (65%); in contrast, after orchiopexy, seminomas represent only 30% of testis tumors.

Because polyorchidism is very uncommon, there is no standard treatment for the condition. Prior to advances in ultrasound technology, it was common practice to remove the supernumerary testicle. Several cases have been described where routine follow-up examinations conducted over a period of years showed that the supernumerary testicle was stable.

A meta-analysis in 2009 suggested removing non-scrotal supernumerary testicles because of the increased risk of cancer, and regular follow-up in the remaining cases to ensure that the supernumerary testicle remains stable.

Torsion is most frequent among adolescents with about 65% of cases presenting between 12–18 years of age. It occurs in about 1 in 4,000 to 1 per 25,000 males per year before 25 years of age; but it can occur at any age, including infancy.

Around 15% of all adult males, up to 35% of men who are evaluated for male infertility, and around 80% of men who are infertile due to some other cause, have varicocele.

Torsion is due to a mechanical twisting process. It is also believed that torsion occurring during fetal development can lead to so-called neonatal torsion or vanishing testis, and is one of the causes of an infant being born with monorchism (one testicle).

To some extent, it is possible to change testicular size. Short of direct injury or subjecting them to adverse conditions, e.g., higher temperature than they are normally accustomed to, they can be shrunk by competing against their intrinsic hormonal function through the use of externally administered steroidal hormones. Steroids taken for muscle enhancement (especially anabolic steroids) often have the undesired side effect of testicular shrinkage.

Similarly, stimulation of testicular functions via gonadotropic-like hormones may enlarge their size. Testes may shrink or atrophy during hormone replacement therapy or through chemical castration.

In all cases, the loss in testes volume corresponds with a loss of spermatogenesis.

Testicular size as a proportion of body weight varies widely. In the mammalian kingdom, there is a tendency for testicular size to correspond with multiple mates (e.g., harems, polygamy). Production of testicular output sperm and spermatic fluid is also larger in polygamous animals, possibly a spermatogenic competition for survival. The testes of the right whale are likely to be the largest of any animal, each weighing around 500 kg (1,100 lb).

Among the Hominidae, gorillas have little female promiscuity and sperm competition and the testes are small compared to body weight (0.03%). Chimpanzees have high promiscuity and large testes compared to body weight (0.3%). Human testicular size falls between these extremes (0.08%).

Testis weight also varies in seasonal breeders like deer and horses. The change is related to changes in testosterone production.

Human testicles are smaller than chimpanzee testicles but larger than gorilla testicles.

This can be due to:

- One testicle not descending into the scrotum during normal embryonic or fetal development (3–4% of 'normal' live births), also known as undescended testis or cryptorchidism. In this case the testis is within the abdominal cavity, somewhere along the normal route of descent – most commonly, within the inguinal canal. Such a testis has an increased risk of malignancy.

- One testicle may disappear during development (the so-called vanishing testis) due to some intrauterine insult. This is thought to be most likely vascular, such as testicular torsion.

- One testicle may have been surgically removed through orchiectomy.

- One testicle may be injured.

Although extremely rare, monorchism has been observed to be characteristic of some animal species, notably in beetles.

Often the greatest concern with respect to varicocele is its effect on male fertility. The relationship between varicocele and infertility is unclear; some men with the condition are fertile, some have sperm that are normal in shape and move normally, but are compromised in function, and some have sperm with abnormal shapes or that do not move well. Theories as to how variocele affects sperm function include damage via excess heat caused by the blood pooling and oxidative stress on sperm (ROS).

Tobacco smoking and mutations in the gene expressing glutathione S-transferase Mu 1 both put men at risk for infertility; these factors may also exacerbate the risk that varicocele will affect fertility.

A ectopic testis is a testicle that, although not an undescended testicle, has taken a non-standard path through the body and ended up in an unusual location.

The positions of the ectopic testis may be: in the lower part of the abdomen, front of thigh, femoral canal, skin of penis or behind the scrotum. The testis is usually developed, and accompanied by an indirect inguinal hernia. It may be divorced from the epididymis which may lie in the scrotum.

A hydrocele testis is not generally thought to affect fertility. However, it may be indicative of other factors that may affect fertility.

Spermatoceles can originate as diverticulum from the tubules found in the head of the epididymis. Sperm formation gradually causes the diverticulum to increase in size, causing a spermatocele. They are due to continuity between the epididymis and tunica vaginalis.

They are also believed to result from epididymitis, physical trauma, or vasectomy. Scarring of any part of the epididymis can cause it to become obstructed and in turn form a spermatocele.

Sex determination and differentiation is generalized with chromosomal sex during fertilization. At early stages, phenotypic sex does not match chromosomal sex—until later during intrauterine development, sexual maturation is reached. During intrauterine development, females change to male with the testes moving down from a blind vaginal pouch with a developing scrotum, as well as a penis which initially resembled a clitoris. What seems like a female phenotype is altered by increased testosterone levels secretion.

Mutations affecting the androgen receptor (AR) gene may cause either complete or partial androgen insensitivity syndrome. Androgen, a hormone used to describe a group of sex steroid hormones, is responsible for affecting male pseudohermaphroditism. The differentiation of the fetus as male takes place during the sixth or seventh week of gestation. The development is directed by the testicular determining factor: the gene SRY (sex determining region on Y chromosome). Throughout 9th to 13th week, the development of a male genitalia is dependent upon the conversion of testosterone to the more potent androgen by the action of 5α-reductase within the target tissues of the genitalia. A type of internal male pseudohermaphroditism is Persistent Müllerian duct syndrome, which is developed through synthesis of Müllerian-inhibiting factor defects. In such instances, duct derivatives are now in 46XY males—this includes the uterus, fallopian tubes, and upper vagina. These individuals with a hernia sac and bowel loops were found with duct derivatives as well as testes.

A study on a male pseudohermaphrodite kitten showed there was a combination of gastrointestinal and urogenital congenital abnormalities. It was confirmed to have type II atresia ani and rectovaginal fistula that is associated with male pseudohermaphroditism.

Microorchidism is a genetic disorder found in males, characterized by abnormally small testes. The condition is associated with (and often secondary to) a number of other genetic disorders, including Klinefelter's Syndrome and Prader-Willi syndrome, as well as other multiple malformation disorders. The degree of abnormality (or otherwise) of the testes can be determined by the use of an orchidometer. In addition, Microorchidism may also occur as a result of shrinkage or atrophy of the testis due to infections like mumps.

Fournier's gangrene ( an aggressive and rapidly spreading infection of the perineum ) usually presents with fever and intense pain. It is a rare condition but fatal if not identified and aggressively treated with a combination of surgical debridement and broad spectrum antibiotics.

Chronic scrotal pain (pain for greater than 3 months) may occur due to a number of underlying conditions. It occurs in 15-19% of people post vasectomy, due to infections such as epididymitis, prostatitis, and orchitis, as well as varicocele, hydrocele, spermatocele, polyarteritis nodosa, testicular torsion, previous surgery and trauma. In 25% of cases the cause is never determined. The pain can persist for a long and indefinite period of time following the vasectomy, in which case it is termed post-vasectomy pain syndrome (PVPS).

During embryogenesis, the testis descends through the inguinal canal, drawing a diverticulum of peritoneum into the scrotum as it descends. This peritoneal tissue is known as the processus vaginalis. Normally, the communication between the processus vaginalis and the peritoneum is obliterated, and the tunica vaginalis is the tissue that remains overlying the testis and the epididymis. Congenital hydrocele results when the processus vaginalis remains patent, allowing fluid from the peritoneum to accumulate in the scrotum.

Small cysts are best left alone, as are larger cysts that are an asymptomatic condition. Only when the cysts are causing discomfort and are enlarging in size, or the patient wants the spermatocele removed, should a spermatocelectomy be considered. Pain may persist even after removal.

Spermatocelectomy can be performed on an outpatient basis, with the use of local or general anesthesia.

A spermatocelectomy will not improve fertility.

Pseudohermaphroditism, or pseudo-hermaphroditism, is an old clinical term for an organism is born with primary sex characteristics of one sex but develops the secondary sex characteristics that are different from what would be expected on the basis of the gonadal tissue (ovary or testis). It can be contrasted with the term true hermaphroditism, which described a condition where testicular and ovarian tissue were present in the same individual. This language has fallen out of favor due to misconceptions and pejorative connotations associated with the terms, and also a shift to nomenclature based on genetics.

The term "male pseudohermaphrodite" was used when a testis is present, and the term "female pseudohermaphrodite" was used when an ovary is present.

In some cases, external sex organs associated with pseudohermaphroditism look intermediate between a typical clitoris and penis. In other cases, the external sex organs have an appearance that would be expected to be seen with the "opposite" gonadal tissue. Because of this, pseudohermaphroditism is sometimes not identified until puberty or adulthood.

Associated conditions include 5-α-reductase deficiency and androgen insensitivity syndrome.

A testicular nubbin is the residual tissue of the human testis after a supposed perinatal vascular accident involving the testicular blood supply. The blood supply of the testis twists (called torsion) thereby cutting off the blood supply to the testis and results in testicular atrophy (shrinking). The nubbin is usually identified in childhood by the absence of a palpable testis in the scrotal sac. The tissue remnant usually includes fibrous tissue and signs of old infarction with hemosiderin deposition identified histologically. There is some disagreement as to whether these should be removed and whether there is a risk of future malignancy. They are typically removed surgically by pediatric urologists or pediatric general surgeons through either a scrotal or inguinal (or both) incision.

Leydig cell hypoplasia (or aplasia) (LCH), also known as Leydig cell agenesis, is a rare autosomal recessive genetic and endocrine syndrome affecting an estimated 1 in 1,000,000 genetic males. It is characterized by an inability of the body to respond to luteinizing hormone (LH), a gonadotropin which is normally responsible for signaling Leydig cells of the testicles to produce testosterone and other androgen sex hormones. The condition manifests itself as pseudohermaphroditism (partially or fully underdeveloped genitalia), hypergonadotropic hypogonadism (decreased or lack of production of sex steroids by the gonads despite high circulating levels of gonadotropins), reduced or absent puberty (lack of development of secondary sexual characteristics, resulting in sexual infantilism if left untreated), and infertility.

Leydig cell hypoplasia does not occur in biological females as they do not have either Leydig cells or testicles. However, the cause of the condition in males, luteinizing hormone insensitivity, does affect females, and because LH plays a role in the female reproductive system, it can result in primary amenorrhea or oligomenorrhea (absent or reduced menstruation), infertility due to anovulation, and ovarian cysts.

A related condition is follicle-stimulating hormone (FSH) insensitivity, which presents with similar symptoms to those of Leydig cell hypoplasia but with the symptoms in the respective sexes reversed (i.e., hypogonadism and sexual infantilism in females and merely problems with fertility in males). Despite their similar causes, FSH insensitivity is considerably less common in comparison to LH insensitivity.

The 2006 Consensus statement on the management of intersex disorders states that individuals with 17β-hydroxysteroid dehydrogenase III deficiency have an intermediate risk of germ cell malignancy, at 28%, recommending that gonads be monitored. A 2010 review put the risk of germ cell tumors at 17%.

The management of 17β-hydroxysteroid dehydrogenase III deficiency can consist, according to one source, of the elimination of gonads prior to puberty, in turn halting masculinization.

Hewitt and Warne state that, children with 17β-hydroxysteroid dehydrogenase III deficiency who are raised as girls often later identify as male, describing a "well known, spontaneous change of gender identity from female to male" that "occurs after the onset of puberty." A 2005 systematic review of gender role change identified the rate of gender role change as occurring in 39–64% of individuals with 17β-hydroxysteroid dehydrogenase III deficiency raised as girls.