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.

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).

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.

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.

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.

A direct inguinal hernia is less common (~25–30% of inguinal hernias) and usually occurs in men over 40 years of age.

Men have an 8 times higher incidence of inguinal hernia than women.

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.

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

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.

90% of ruptured testes are successfully repaired when treated surgically within 72 hours; the percentage of successful treatment drops to 45% after this period. Though not typically fatal, testicular rupture can cause hypogonadism, low self-esteem, and infertility.

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.

Links between maternal smoking and TDS are tenuous, but there are stronger associations between maternal alcohol consumption and incidences of cryptorchidism in sons. Smoking does however affect the growth of a fetus, and low birth weight is shown to increase the likelihood of all the disorders encompassed by TDS. Maternal obesity, resulting in gestational diabetes, has also been shown to be a risk factor for impaired testes development and TDS symptoms in sons.

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.

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.

Secondary hydroceles due to testicular diseases can be the result of cancer, trauma (such as a hernia), or orchitis (inflammation of testis), and can also occur in infants undergoing peritoneal dialysis. A hydrocele is not a cancer but it should be excluded clinically if a presence of a testicular tumor is suspected, however, there are no publications in the world literature that report a hydrocele in association with testicular cancer. Secondary hydrocele is most frequently associated with acute or chronic epididymo-orchitis. It is also seen with torsion of the testis and with some testicular tumors. A secondary hydrocele is usually lax and of moderate size: the underlying testis is palpable. A secondary hydrocele subsides when the primary lesion resolves.

- Acute/chronic epididymo-orchitis

- Torsion of testis

- Testicular tumor

- Hematocele

- Filarial hydrocele

- Post herniorrhaphy

- Hydrocele of an hernial sac

A potential testicular rupture should be evaluated with ultrasound imaging. Testicular rupture is treated with surgery, though the procedure performed depends on the magnitude of the injury and the salvageability of the tissue. An orchiectomy - removal of the affected testis - is done when the testis is not salvageable and leads to reduced semen quality and higher rates of endocrine dysfunction than repair of salvageable tissue.

Surgical correction of inguinal hernias is called a hernia repair. It is not recommended in minimally symptomatic hernias, for which watchful waiting is advised, due to the risk of post herniorraphy pain syndrome. Surgery is commonly performed as outpatient surgery. There are various surgical strategies which may be considered in the planning of inguinal hernia repair. These include the consideration of mesh use (e.g. synthetic or biologic), open repair, use of laparoscopy, type of anesthesia (general or local), appropriateness of bilateral repair, etc. Laparoscopy is most commonly used for non-emergency cases, however, a minimally invasive open repair may have a lower incidence of post-operative nausea and mesh associated pain. During surgery conducted under local anaesthesia, the patient will be asked to cough and strain during the procedure to help in demonstrating that the repair is without tension and sound.

Constipation after hernia repair results in strain to evacuate the bowel causing pain, and fear that the sutures may rupture. Opioid analgesia makes constipation worse. Promoting an easy bowel motion is important post-operatively.

Surgical correction is always recommended for inguinal hernias in child.

Emergency surgery for incarceration and strangulation carry much higher risk than planned, "elective" procedures. However, the risk of incarceration is low, evaluated at 0.2% per year. On the other hand, surgery has a risk of inguinodynia (10-12%), and this is why males with minimal symptoms are advised to watchful waiting. However, if they experience discomfort while doing physical activities or they routinely avoid them by the fear of pain, they should seek surgical evaluation. For female patients, surgery is recommended even for asymptomatic patients.

Surgery (orchiopexy) to retrieve the testes and position them in the scrotum is the primary treatment. Occasionally they are unsalvageable if located high in the retroperitoneum. During this surgery, the uterus is usually removed and attempts made to dissect away Müllerian tissue from the vas deferens and epididymis to improve the chance of fertility. If the person has male gender identity himself and the testes cannot be retrieved, testosterone replacement will be usually necessary at puberty should the affected individual choose to pursue medical attention. Lately, laparoscopic hysterectomy is offered to patients as a solution to both improve the chances of fertility and to prevent the occurrences of neoplastic tissue formation.

PMDS type I results from mutations of the gene ("AMH") for AMH on chromosome 19p3.3.

PMDS type II results from mutations of the gene ("AMH-RII") for the AMH receptor on 12q13.

Both types of disorders are inherited as autosomal recessive conditions with expression usually limited to XY offspring.

Pre-testicular factors refer to conditions that impede adequate support of the testes and include situations of poor hormonal support and poor general health including:

- Hypogonadotropic hypogonadism due to various causes

- Obesity increases the risk of hypogonadotropic hypogonadism. Animal models indicate that obesity causes leptin insensitivity in the hypothalamus, leading to decreased Kiss1 expression, which, in turn, alters the release of gonadotropin-releasing hormone (GnRH).

- Undiagnosed and untreated coeliac disease (CD). Coeliac men may have reversible infertility. Nevertheless, CD can present with several non-gastrointestinal symptoms that can involve nearly any organ system, even in the absence of gastrointestinal symptoms. Thus, the diagnosis may be missed, leading to a risk of long-term complications. In men, CD can reduce semen quality and cause immature secondary sex characteristics, hypogonadism and hyperprolactinaemia, which causes impotence and loss of libido. The giving of gluten free diet and correction of deficient dietary elements can lead to a return of fertility. It is likely that an effective evaluation for infertility would best include assessment for underlying celiac disease, both in men and women.

- Drugs, alcohol

- Strenuous riding (bicycle riding, horseback riding)

- Medications, including those that affect spermatogenesis such as chemotherapy, anabolic steroids, cimetidine, spironolactone; those that decrease FSH levels such as phenytoin; those that decrease sperm motility such as sulfasalazine and nitrofurantoin

- Genetic abnormalities such as a Robertsonian translocation

Exposure of a male fetus to substances that disrupt hormone systems, particularly chemicals that inhibit the action of androgens (male sex hormones) during the development of the reproductive system, has been shown to cause many of the characteristic TDS disorders. These include environmental estrogens and anti-androgens found in food and water sources that have been contaminated with synthetic hormones and pesticides used in agriculture. In historical cases, medicines given to pregnant women, like diethylstilbestrol (DES), have caused many of the features of TDS in fetuses exposed to this chemical during gestation. The impact of environmental chemicals is well documented in animal models. If a substance affects Sertoli and Leydig cell differentiation (a common feature of TDS disorders) at an early developmental stage, germ cell growth and testosterone production will be impaired. These processes are essential for testes descent and genitalia development, meaning that genital abnormalities like cryptorchidism or hypospadias may be present from birth, and fertility problems and TGCC become apparent during adult life. Severity or number of disorders may therefore be dependent on the timing of the environmental exposure. Environmental factors can act directly, or via epigenetic mechanisms, and it is likely that a genetic susceptibility augmented by environmental factors is the primary cause of TDS.

There is increasing evidence that the harmful products of tobacco smoking may damage the testicles and kill sperm, but their effect on male fertility is not clear. Some governments require manufacturers to put warnings on packets. Smoking tobacco increases intake of cadmium, because the tobacco plant absorbs the metal. Cadmium, being chemically similar to zinc, may replace zinc in the DNA polymerase, which plays a critical role in sperm production. Zinc replaced by cadmium in DNA polymerase can be particularly damaging to the testes.

A hydrocele can be produced in four ways:

- by excessive production of fluid within the sac, e.g. secondary hydrocele

- through defective absorption of fluid

- by interference with lymphatic drainage of scrotal structures as in case of elephantiasis

- by connection with a hernia of the peritoneal cavity in the congenital variety, which presents as hydrocele of the cord

Orchitis is not rare in bulls and rams.

It has also been described in roosters.

Many people are managed through day surgery centers, and are able to return to work within a week or two, while intense activities are prohibited for a longer period. People who have their hernias repaired with mesh often recover within a month, though pain can last longer. Surgical complications include pain that lasts more than three months, surgical site infections, nerve and blood vessel injuries, injury to nearby organs, and hernia recurrence. Pain that lasts more than three months occurs in about 10% of people following hernia repair.