Symptoms of an ingrown nail include pain along the margins of the nail (caused by hypergranulation that occurs around the aforementioned margins), worsening of pain when wearing tight footwear, and sensitivity to pressure of any kind, even the weight of bedsheets. Bumping of an affected toe can produce sharp and even excruciating pain as the tissue is punctured further by the nail. By the very nature of the condition, ingrown nails become easily infected unless special care is taken early to treat the condition by keeping the area clean. Signs of infection include redness and swelling of the area around the nail, drainage of pus and watery discharge tinged with blood. The main symptom is swelling at the base of the nail on the ingrowing side (though it may be both sides).

Onychocryptosis should not be confused with a similar nail disorder, convex nail, nor with other painful conditions such as involuted nails, nor with the presence of small corns, callus or debris down the nail sulci (grooves on either side).

"Onycholysis" is from "onycho-", from Ancient Greek ὄνυξ "ónuks", meaning "nail", and Ancient Greek λύσις "lúsis", meaning "a loosening".

An ingrown nail (also known as onychocryptosis from ὄνυξ ("onyx", "nail") + κρυπτός ("kryptos", "hidden") or unguis incarnates) is a common form of nail disease. It is an often painful condition in which the nail grows so that it cuts into one or both sides of the paronychium or nail bed.

The common opinion is that the nail enters inside the paronychium, but an ingrown toenail can simply be overgrown toe skin. The condition starts from a microbial inflammation of the paronychium, then a granuloma, which results in a nail buried inside of the granuloma. While ingrown nails can occur in the nails of both the hands and the feet , they occur most commonly with the toenails.

A true ingrown toenail is caused by the actual penetration of flesh by a sliver of nail.

Onycholysis is a common medical condition characterized by the painless detachment of the nail from the nail bed, usually starting at the tip and/or sides. On the hands, it occurs particularly on the ring finger but can occur on any of the fingernails. It may also happen to toenails.

Onycholysis can occur in many conditions, including psoriasis. In thyrotoxicosis it is thought to be due to sympathetic overactivity. It may also be seen in infections or trauma.

"Distribution" refers to how lesions are localized. They may be confined to a single area (a patch) or may exist in several places. Some distributions correlate with the means by which a given area becomes affected. For example, contact dermatitis correlates with locations where allergen has elicited an allergic immune response. Varicella zoster virus is known to recur (after its initial presentation as chicken pox) as herpes zoster ("shingles"). Chicken pox appears nearly everywhere on the body, but herpes zoster tends to follow one or two dermatomes; for example, the eruptions may appear along the bra line, on either or both sides of the patient.

- Generalized

- Symmetric: one side mirrors the other

- Flexural: on the front of the fingers

- Extensor: on the back of the fingers

- Intertriginous: in an area where two skin areas may touch or rub together

- Morbilliform: resembling measles

- Palmoplantar: on the palm of the hand or bottom of the foot

- Periorificial: around an orifice such as the mouth

- Periungual/subungual: around or under a fingernail or toenail

- Blaschkoid: following the path of Blaschko's lines in the skin

- Photodistributed: in places where sunlight reaches

- Zosteriform or dermatomal: associated with a particular nerve

Melanonychia is a black or brown pigmentation of the normal nail plate, and may be present as a normal finding on many digits in African-Americans, as a result of trauma, systemic disease, or medications, or as a postinflammatory event from such localized events as lichen planus or fixed drug eruption.

There are two types, longitudinal and transverse melanonychia. Longitudinal melanonychia may be a sign of subungual melanoma (acral lentiginous melanoma), although there are other diagnoses such as chronic paronychia, onychomycosis, subungual hematoma, pyogenic granuloma, glomus tumour, subungual verruca, mucous cyst, subungual fibroma, keratoacanthoma, carcinoma of the nail bed, and subungual exostosis.

Dermoodontodysplasia is dental problems, trichodysplasia, and nail and skin problems.

"Configuration" refers to how lesions are locally grouped ("organized"), which contrasts with how they are distributed (see next section).

- Agminate: in clusters

- Annular or circinate: ring-shaped

- Arciform or arcuate: arc-shaped

- Digitate: with finger-like projections

- Discoid or nummular: round or disc-shaped

- Figurate: with a particular shape

- Guttate: resembling drops

- Gyrate: coiled or spiral-shaped

- Herpetiform: resembling herpes

- Linear

- Mammillated: with rounded, breast-like projections

- Reticular or reticulated: resembling a net

- Serpiginous: with a wavy border

- Stellate: star-shaped

- Targetoid: resembling a bullseye

- Verrucous: wart-like

Onychomatricoma is a cutaneous condition characterized by a distinctive tumor of the nail matrix.

This nail disease can mimic many nail problems and should be examined and biopsied by a dermatologist. In particular, a main concern is the malignant and destructive potential that may exist.

Pterygium inversum unguis (also known as "Pterygium inversus unguis," and "Ventral pterygium") is characterized by the adherence of the distal portion of the nailbed to the ventral surface of the nail plate. The condition may be present at birth or acquired, and may cause pain with manipulation of small objects, typing, and close manicuring of the nail.

Longitudinal leukonychia is far less common and features smaller 1mm white longitudinal lines visible under the nail plate. It may be associated with Darier's disease.

A hangnail is a small, torn piece of skin, more specifically eponychium or paronychium, next to a fingernail or toenail.

Also known as "true" leukonychia, this is the most common form of leukonychia, in which small white spots appear on the nails. Picking and biting of the nails are a prominent cause in young children and nail biters. Besides parakeratosis, air that is trapped between the cells may also cause this appearance. It is also caused by trauma. In most cases, when white spots appear on a single or a couple of fingers or toes, the most common cause is injury to the base (matrix) of the nail. When this is the case, white spots disappear after around eight months, which is the amount of time necessary for nails to regrow completely. The pattern and number of spots may change as the nail grows.

Beau's lines are deep grooved lines that run from side to side on the fingernail or the toenail. They may look like indentations or ridges in the nail plate. This condition of the nail was named by a French physician, Joseph Honoré Simon Beau (1806–1865), who first described it in 1846.

Beau's lines are horizontal, going across the nail, and should not be confused with vertical ridges going from the bottom (cuticle) of the nail out to the fingertip. These vertical lines are usually a natural consequence of aging and are harmless. Beau's lines should also be distinguished from Muehrcke's lines of the fingernails. While Beau's lines are actual ridges and indentations in the nail plate, Muehrcke lines are areas of hypopigmentation without palpable ridges; they affect the underlying nail bed, and not the nail itself. Beau's lines should also be distinguished from Mees' lines of the fingernails, which are areas of discoloration in the nail plate.

There are several causes of Beau's lines. It is believed that there is a temporary cessation of cell division in the nail matrix. This may be caused by an infection or problem in the nail fold, where the nail begins to form, or it may be caused by an injury to that area. Some other reasons for these lines include trauma, coronary occlusion, hypocalcaemia, and skin disease. They may be a sign of systemic disease, or may also be caused by an illness of the body, as well as drugs used in chemotherapy or malnutrition. Beau's lines can also be seen one to two months after the onset of fever in children with Kawasaki disease.

Human nails grow at a rate which varies with many factors: age, and the finger or toe in question as well as nutrition. However, typically in healthy populations fingernails grow at about 0.1mm/day and toenails at about 0.05mm/day. With this in mind the date of the stress causing Beau's lines and other identifiable marks on nails can be estimated. As the nail grows out, the ridge visibly moves upwards toward the nail edge. When the ridge reaches the nail edge, the fingertips can become quite sore due to the mis-shapen nail pressing into the flesh deeper than usual, exposing the sensitive nail bed (the quick) at the nail edge.

A researcher found Beau's lines in the fingernails of two of six divers following a deep saturation dive to a pressure equal to 305 meters of sea water, and in six of six divers following a similar dive to 335 meters. They have also been seen in Ötzi the Iceman.

Hapalonychia, also known as egg-shell nail, is a condition in which the top of a toe or finger nail becomes soft and thin, causing it to bend or break. This condition can manifest as a result of genetic discrepancy and may also occur due to malnutrition and debility. Nails are often a representation of the internal nutritional of health. Although overall well-being is not typically determined by nail health, fissures, breaks and calcium spots are minor indications of inner health. Hapalonychia may also be attributed to myxedema, rheumatoid arthritis, anorexia, bulimia, Hansen's disease, Raynaud phenomenon, oral retinoid therapy, or radiodermatitis.

In racquet nails (also known as brachyonychia, nail en raquette, and racquet thumb), the nail plate is flattened, the end of the thumb is widened and flattened, and the distal phalanx is abnormally short. In racquet nails, the width of the nail bed and nail plate is greater than their length. The condition is painless and asymptomatic.

Racquet nails usually occur on one or both thumbs, but may appear on all fingers and toes. It may be genetic, inherited as an autosomal-dominant trait, or acquired. Genetic disorders associated with racquet nail include Larsen syndrome, Brooke–Spiegler syndrome, Rubinstein–Taybi syndrome, Hajdu–Cheney syndrome, cartilage–hair hypoplasia, pycnodysostosis, acrodysostosis, and brachydactylia.

Acquired racquet nail is associated with acroosteolysis and psoriatic arthropathy. Acquired racquet nail may also be diagnostic of bone resorption in hyperparathyroidism.

Racquet nail often presents with other nail conditions such as onycholysis, koilonychia, pachyonychia, Muehrcke's lines, leuconychia, and half-and-half nails. Cosmetic surgery can be used to improve the appearance of the nails.

Longitudinal erythronychia presents with longitudinal red bands in the nail plate that commence in the matrix and extend to the point of separation of the nail plate and nailbed, and may occur on multiple nails with inflammatory conditions such as lichen planus or Darier's disease.

Hook nail is a bowing of the nail bed due to a lack of support from the short bony phalanx.

Nail inspection can give hints to the internal condition of the body as well.

Nail disease can be very subtle and should be evaluated by a dermatologist with a focus in this particular area of medicine. A nail technician may be the first to note a subtle change in nail health.

Hangnails can become infected and cause paronychia, a type of skin infection that occurs around the nails. Treatments for paronychia vary with severity, but may include soaking in hot salty water, the use of oral antibiotic medication, or clinical lancing. Paronychia itself rarely results in further complications but can lead to abscess, permanent changes to the shape of the nail or the spread of infection.

Neoplasms of the nailbed may often present with paronychia, ingrown nail, onycholysis, pyogenic granuloma, nail-plate dystrophy, longitudinal erythronychia, bleeding, and discolorations. There are various benign and malignant neoplasms that may occur in or overlying the nail matrix and in the nailbed, and symptoms may include pain, itching, and throbbing.

Benign tumors of the nails include verruca, pyogenic granuloma, fibromas, nevus cell nevi, myxoid cysts, angiofibromas (Koenen tumors), and epidermoid cysts.

Squamous cell carcinoma of the nailbed is uncommon, and often mistaken for a pyogenic granuloma initially. Subungual melanoma is frequently diagnosed late in the course of growth.

A nail disease or onychosis is a disease or deformity of the nail. Although the nail is a structure produced by the skin and is a skin appendage, nail diseases have a distinct classification as they have their own signs and symptoms which may relate to other medical conditions. Some nail conditions that show signs of infection or inflammation may require medical assistance.

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.

Apparent leukonychia is a cutaneous condition characterized by white discoloration of the nail that fades with pressure.

Psoriatic nails are characterized by a translucent discolouration in the nail bed that resembles a drop of oil beneath the nail plate. Early signs that may accompany the "oil drop" include thickening of the lateral edges of the nail bed with or without resultant flattening or concavity of the nail; separation of the nail from the underlying nail bed, often in thin streaks from the tip-edge to the cuticle; sharp peaked "roof-ridge" raised lines from cuticle to tip; or separation of superficial layers of the nail followed by loss of patches of these superficial layers, leaving thin red nails beneath; or nail pitting–punctate changes along the nail plate surface.