Actinic keratosis is very common, with an estimated 14% of dermatology visits related to AKs. It is seen more often in fair-skinned individuals, and rates vary with geographical location and age. Other factors such as exposure to ultraviolet (UV) radiation, certain phenotypic features, and immunosuppression can also contribute to the development of AKs.

Men are more likely to develop AK than women, and the risk of developing AK lesions increases with age. These findings have been observed in multiple studies, with numbers from one study suggesting that approximately 5% of women ages 20–29 develop AK compared to 68% of women ages 60–69, and 10% of men ages 20–29 develop AK compared to 79% of men ages 60–69.

Geography seems to play a role in the sense that individuals living in locations where they are exposed to more UV radiation throughout their lifetime have a significantly higher risk of developing AK. Much of the literature on AK comes from Australia, where prevalence of AK is estimated at 40–50% in adults over 40, as compared to the United States and Europe, where prevalence is estimated at under 11–38% in adults. One study found that those who immigrated to Australia after age 20 had fewer AKs than native Australians in all age groups.

Melanin is a pigment in the epidermis that functions to protect keratinocytes from the damage caused by UV radiation; it is found in higher concentration in the epidermis of darker-skinned individuals, affording them protection against the development of AKs. Fair-skinned individuals have a significantly increased risk of developing AKs when compared to olive skinned individuals (odds ratios of 14.1 and 6.5, respectively), and AKs are uncommon in dark-skinned African Americans. Other phenotypic features seen in fair-skinned individuals that are associated with an increased propensity to develop AKs include:

- Freckling

- Light hair color

- Propensity to sunburn

- Inability to tan

CCCA tends to present itself in the 20s and progresses over 20–30 years. One should consider this diagnosis in African Americans with what appears to be a female-pattern hair loss.

Worldwide, KP affects an estimated 30-50% of the adult population and approximately 50-80% of all adolescents. It is more common in women than in men, and is often present in otherwise healthy individuals. The skin condition is prevalent in persons of all ethnicities. No particular ethnicity is at higher risk for developing keratosis pilaris. Although keratosis pilaris may manifest in persons of any age, it usually appears within the first decade of life and is more common in young children. In most cases, the condition gradually improves before age 30, however it can persist longer.

Usually, a common form of treatment for the condition is a type of hand cream which moisturises the hard skin. However, currently the condition is incurable.

Razor burn is a less serious condition caused by shaving, characterized by mild to moderate redness and irritation on the surface of the skin. Unlike PFB, it is usually transient and there is no infection involved.

There is also a condition called "folliculitis barbae". The difference between the two is the cause of the inflammation in the hair follicles. Folliculitis barbae is caused by viral or bacterial infections, where pseudofolliculitis is caused by irritation from shaving and ingrown hairs.

A related condition, pseudofolliculitis nuchae, occurs on the back of the neck, often along the posterior hairline, when curved hairs are cut short and allowed to grow back into the skin. Left untreated, this can develop into acne keloidalis nuchae, a condition where hard, dark keloid-like bumps form on the neck. Both occur frequently in black men in the military, where it is so common that services often have widely known protocols for management

Treatment of manifestations: special hair care products to help manage dry and sparse hair; wigs; artificial nails; emollients to relieve palmoplantar hyperkeratosis.

Seborrheic keratosis is the most common benign skin tumor. Incidence increases with age. There is less prevalence in people with darker skin. In large-cohort studies, 100% of the patients over age 50 had at least one seborrheic keratosis. Onset is usually in middle age, although they are common in younger patients too—found in 12% of 15-year-olds to 25-year-olds—making the term "senile keratosis" a misnomer.

Many conditions affect the human integumentary system—the organ system covering the entire surface of the body and composed of skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, the hairs occur in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Nourishment is provided to these layers by diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis. This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface. In normal skin, the rate of production equals the rate of loss; about two weeks are needed for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.

The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis. The superficial papillary dermis with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and ground substance. Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.

The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus. The main cellular component of this tissue is the adipocyte, or fat cell. The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on. Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow). Diagnosis of many conditions often also requires a skin biopsy which yields histologic information that can be correlated with the clinical presentation and any laboratory data.

Ectodermal dysplasia with corkscrew hairs is a skin condition with salient features including exaggerated pili torti, scalp keloids, follicular plugging, keratosis pilaris, xerosis, eczema, palmoplantar keratoderma, syndactyly, onchodysplasia, and conjunctival neovascularization.

Epidermolytic palmoplantar keratoderma has been associated with keratin 9 and keratin 16.

Nonepidermolytic palmoplantar keratoderma has been associated with keratin 1 and keratin 16.

There is controversy over whether auricular hypertrichosis is a Y-linked or autosomal trait, or perhaps both (in different families). It was proposed also that this phenotype results from the interaction of two loci, one on the homologous part of the X and Y and one on the nonhomologous sequence of the Y.

Lee et al. (2004), by Y-chromosomal DNA binary-marker haplotyping, suggested that a cohort of southern Indian hairy-eared males carried Y chromosomes from many haplogroups of the Y-phylogeny. According to a hypothesis of Y linkage, it would require multiple independent mutations within a single population. No significant difference between the Y-haplogroup frequencies of hairy-eared males and those of a geographically matched control sample of unaffected males was established. They concluded that the auricular hypertrichosis is not Y-linked in southern India, but it is unlikely to be same in any population.

Ingrown hair is a condition where hair curls back or grows sideways into the skin. The condition is most prevalent among people who have coarse or curly hair. It may or may not be accompanied by an infection of the hair follicle (folliculitis) or "razor bumps" (pseudofolliculitis barbae), which vary in size. While ingrown hair most commonly appears in areas where the skin is shaved or waxed (beard, legs, pubic region), it can appear anywhere. Anything which causes the hair to be broken off unevenly with a sharp tip can cause ingrown hairs. Ingrown hairs are also caused because of lack of natural exfoliation in the skin.

The best form of prevention is to determine whether shaving or waxing irritates the skin and hair worse.When shaving, there are a few precautions that can be taken to prevent ingrown hairs including proper shaving techniques and preparation of the skin before shaving. When shaving, applying the proper amount of lubrication (in the form of shaving cream, gel, or soap) is important to prevent the hair from being forced underneath the surface of the skin. Also the application of too much force with a razor can contribute to hair that is cut shorter than the surrounding dermis. Using a beard trimmer at the lowest setting (1 mm or 0.5 mm) instead of shaving is an effective alternative.

Alternatively, ingrown hair can be prevented by removing the hair permanently, e.g. by laser hair removal or hair removal through electrolysis.

Keratosis pilaris atrophicans faciei (also known as "Folliculitis rubra," "Keratosis pilaris rubra atrophicans faciei," "Lichen pilare," "Lichen pilaire ou xerodermie pilaire symetrique de la face," "Ulerythema ophryogenes," and "Xerodermie pilaire symetrique de la face") begins in infancy as follicular papules with perifollicular erythema. Initially, the lesions are restricted to the lateral eyebrows, but with time spread to involve the cheeks and forehead, and may also be associated with keratosis pilaris on the extremities and buttocks.

Porokeratosis is a specific disorder of keratinization that is characterized histologically by the presence of a cornoid lamella, a thin column of closely stacked, parakeratotic cells extending through the stratum corneum with a thin or absent granular layer.

Monilethrix (also referred to as beaded hair) is a rare autosomal dominant hair disease that results in short, fragile, broken hair that appears beaded. It comes from the Latin word for necklace ("monile") and the Greek word for hair ("thrix").

Pseudofolliculitis barbae (PFB), also known as barber's itch, folliculitis barbae traumatica, razor bumps, scarring pseudofolliculitis of the beard, and shave bumps, is a medical term for persistent irritation caused by shaving. Pseudofolliculitis barbae was first described in 1956.

In 2016, interferon gamma/CXCL10 axis was hypothesized to be a target for treatments that reverse inflammation. Apremilast is undergoing investigation as a potential treatment .

Monilethrix is caused by mutations affecting the genes KRTHB1 (KRT81), KRTHB3 (KRT83), or KRTHB6 (KRT86) which code for type II hair cortex keratins. The disorder is inherited in an autosomal dominant manner. This means that the defective gene(s) responsible for the disorder is located on an autosome, and only one copy of the gene is sufficient to cause the disorder, when inherited from a parent who has the disorder.

Severe congenital onychogryphosis affecting all twenty nailbeds has been recorded in two families who exhibit the dominant allele for a certain gene. Congenital onychogryphosis of the fifth toe (the little toe) is fairly common, but asymptomatic and seldom brought to the attention of medical professionals. Rather, it is brought to the attention of manicurists who routinely file the clawed toenail flat.

Pustulosis palmaris et plantaris (also known as pustulosis of palms and soles, palmoplantar pustulosis, persistent palmoplantar pustulosis, pustular psoriasis of the Barber type, and pustular psoriasis of the extremities) is a chronic recurrent pustular dermatosis (that is, a pustulosis or pustular psoriasis) localized on the palms and soles only, characterized histologically by intraepidermal pustules filled with neutrophils. It can occur as part of the SAPHO syndrome.

The overall prevalence of lichen planus in the general population is about 0.1–4.0%. It generally occurs more commonly in females, in a ratio of 3:2, and most cases are diagnosed between the ages of 30 and 60, but it can occur at any age.

Oral lichen planus is relatively common, It is one of the most common mucosal diseases. The prevalence in the general population is about 1.27–2.0%, and it occurs more commonly in middle aged people. OLP in children is rare. About 50% of females with oral lichen planus were reported to have undiagnosed vulvar lichen planus.

The mechanism of pathology of CCCA has only been postulated and not proven. However, one theory involves pressure exerted on the internal root sheath leading to damage, which leads to the recruitment of inflammatory cells and the end result of scarring. African Americans are found to be at increased risk either because of the curled hair shaft, distinct styling practices, moisturizing hair products, or chemical processing techniques (especially in the youth population). Some have hypothesized that CCCA represents an end stage of traction alopecia. However, this theory does not ring true as many patients lack a report of traction hairstyling.

HED2 is suspected after infancy on the basis of physical features in most affected individuals. GJB6 is the only gene known to be associated with HED2. Targeted mutation analysis for the four most common GJB6 mutations is available on a clinical basis and detects mutations in approximately 100% of affected individuals. Sequence analysis is also available on a clinical basis for those in whom none of the four known mutations is identified.