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.

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

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.

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.

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).

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.

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.

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.

Onychorrhexis (from the Greek words ὄνυχο- "ónycho-", "nail" and ῥῆξις "rhexis", "bursting"), also known as brittle nails, is a brittleness with breakage of finger or toenails that may result from excessive strong soap and water exposure, nail polish remover, hypothyroidism, anemia, anorexia nervosa or bulimia, or after oral retinoid therapy. Onychorrhexis affects up to 20% of the population.

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.

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.

Green nail syndrome, also known as chloronychia, is a paronychial infection caused due to "Pseudomonas aeruginosa" that can develop in individuals whose hands are frequently submerged in water. It may also occur as transverse green stripes that are ascribed to intermittent episodes of infection.

Dysplastic nails are a cutaneous condition, and may be a subtle finding of ridging, flaking, or poor growth of the nails, or more diffuse with nearly complete loss of nails. This condition may be seen in a number of syndromes, including Dyskeratosis congenita and Nail–patella syndrome.

Onychomadesis is a periodic idiopathic shedding of the nails beginning at the proximal end, possibly caused by the temporary arrest of the function of the nail matrix.One cause in children is hand foot and mouth disease. This generally resolves without complication.

Onychomadesis can also occur if the nail is damaged or suffers a loss of blood supply, i.e. due to a bruise. The nailbed turns black, and the nail drops off shortly afterwards. It takes around 6 to 12 months to regrow.

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.

Drug-induced nail changes are caused by drug administration which may result in a large number of nail abnormalities, usually with multiple nails being affected at the same time.

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.

They are typically white bands traversing the width of the nail. As the nail grows they move towards the end, and finally disappear when trimmed.

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.

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.

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.

Tooth and nail syndrome (also known as "Hypodontia with nail dysgenesis," and "Witkop syndrome") is a rare disorder, first described in 1965, characterized by nails that are thin, small, and friable, and which may show koilonychia at birth.

It is associated with "MSX1".

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.

In season 6 of House MD in the episode 12 titled Remorse, House diagnoses his patient with Wilson's Disease in absence of Kayser-Fleischer rings by removing the nail polish to note the blue nails.

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