Tinea capitis (also known as "herpes tonsurans", "ringworm of the hair", "ringworm of the scalp", "scalp ringworm", and "tinea tonsurans") is a cutaneous fungal infection (dermatophytosis) of the scalp. The disease is primarily caused by dermatophytes in the genera Trichophyton and Microsporum that invade the hair shaft. The clinical presentation is typically single or multiple patches of hair loss, sometimes with a 'black dot' pattern (often with broken-off hairs), that may be accompanied by inflammation, scaling, pustules, and itching. Uncommon in adults, tinea capitis is predominantly seen in pre-pubertal children, more often boys than girls.

At least eight species of dermatophytes are associated with tinea capitis. Cases of Trichophyton infection predominate from Central America to the United States and in parts of Western Europe. Infections from Microsporum species are mainly in South America, Southern and Central Europe, Africa and the Middle East. The disease is infectious and can be transmitted by humans, animals, or objects that harbor the fungus. The fungus can also exist in a carrier state on the scalp, without clinical symptomatology. Treatment of tinea capitis requires an oral antifungal agent; griseofulvin is the most commonly used drug, but other newer antimycotic drugs, such as terbinafine, itraconazole, and fluconazole have started to gain acceptance.

It may appear as thickened, scaly, and sometimes boggy swellings, or as expanding raised red rings (ringworm). Common symptoms are severe itching of the scalp, dandruff, and bald patches where the fungus has rooted itself in the skin. It often presents identically to dandruff or seborrheic dermatitis. The highest incidence in the United States of America is in American boys of school age.

There are three type of tinea capitis, microsporosis, trichophytosis, and favus; these are based on the causative microorganism, and the nature of the symptoms. In microsporosis, the lesion is a small red papule around a hair shaft that later becomes scaly; eventually the hairs break off 1–3 mm above the scalp. This disease used to be caused primarily by Microsporum audouinii, but in Europe, M. canis is more frequently the causative fungus. The source of this fungus is typically sick cats and kittens; it may be spread through person to person contact, or by sharing contaminated brushes and combs. In the United States, Trichophytosis is usually caused by Trichophyton tonsurans, while T. violaceum is more common in Eastern Europe, Africa, and India. This fungus causes dry, non-inflammatory patches that tend to be angular in shape. When the hairs break off at the opening of the follicle, black dots remain. Favus is caused by T. schoenleinii, and is endemic in South Africa and the Middle East. It is characterized by a number of yellowish, circular, cup-shaped crusts (scutula) grouped in patches like a piece of honeycomb, each about the size of a split pea, with a hair projecting in the center. These increase in size and become crusted over, so that the characteristic lesion can only be seen around the edge of the scab.

From the site of inoculation, the fungus grows down into the stratum corneum, where it invades keratin. Dermatophytes are unique in that they produce keratinase, which enables them to use keratin as a nutrient source. Infected hairs become brittle, and after three weeks, the clinical presentation of broken hairs is evident.

There are three types of infection:

Ectothrix: Characterized by the growth of fungal spores (arthroconidia) on the exterior of the hair shaft.  Infected hairs usually fluoresce greenish-yellow under a Wood's lamp (blacklight). Associated with Microsporum canis, Microsporum gypseum, Trichophyton equinum, and Trichophyton verrucosum.

Endothrix: Similar to ectothrix, but characterized by arthroconidia restricted to the hair shaft, and restricted to anthropophilic bacteria.  The cuticle of the hair remains intact and clinically this type does not have fluorescence.  Associated with Trichophyton tonsurans and Trichophyton violaceum, which are anthropophilic.