An ingrown nail, also known as onychocryptosis, from Ancient Greek ὄνυξ (ónux), meaning "nail", and κρυπτός (kruptós), meaning "hidden", 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. While ingrown nails can occur in the nails of both the hands and the feet, they occur most commonly with the toenails (as opposed to fingernails).^{[citation needed]}

A common misconception is that the cause of an ingrown toenail is the nail growing into the paronychium, but it can also be caused by overgrown toe skin. The condition is caused by a microbial inflammation of the paronychium causing a granuloma within which the nail is buried. A true ingrown toenail is caused by actual penetration of flesh by a sliver of toenail.

Symptoms include pain along the margins of the nail caused by hypergranulation, worsening of pain when wearing tight footwear, and sensitivity to pressure of any kind (in some cases this pressure can be as light as the weight of bedsheets). Bumping of an affected toe can cause pain as the nail's surrounding tissue is punctured further. Ingrown nails can become easily infected unless care is taken early to treat the condition.

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 present on both sides).

Onychocryptosis should not be confused with irregular nail growth patterns such as convex nails, involuted nails, or pincer nails. It also should not be confused with the presence of small corns, callus, or debris down the nail sulci.

The main contributor to onychocryptosis is footwear, particularly ill-fitting shoes with inadequate toe box room and tight stockings that apply pressure to the top or side of the foot. Other factors may include the damp atmosphere of enclosed shoes, which soften the nail plate and cause swelling on the epidermal keratin (eventually increasing the convex arch permanently), genetics, trauma, and disease. Improper cutting of the nail may cause the nail to cut into the side-fold skin from growth and impact, whether or not the nail is truly "ingrown". The nail bends inwards or upwards depending on the angle of its cut. If the cutting tool, such as scissors, is at an attitude in which the lower blade is closer to the toe than the upper blade, the toenail will tend to grow upwards from its base, and vice versa. The process is visible along the nail as it grows, appearing as a warp advancing to the end of the nail. The upper corners turn more easily than the center of the nail tip. Holding the tool at the same angle for all nails may induce these conditions; as the nail turns closer to the skin, it becomes harder to fit the lower blade in the right attitude under the nail. When cutting a nail, it is not just the correct angle that is important, but also how short it is cut. A shorter cut will bend the nail more, unless the cut is even on both top and bottom of the nail.

Causes may include:

One study compared patients with ingrown toenails to healthy controls and found no difference in the shape of toenails between those of patients and of the control group. The study suggested that treatment should not be based on the correction of a non-existent nail deformity. In some cases, however, there is nail deformity.