Abnormalities of the hair shaft are varied and often confusing. They do require accurate recognition, which can be helpful in the diagnosis and management of a hair disorder, or in the detection of underlying disease. A reliable evaluation of hair shaft abnormality is dependent on familiarity with the vast structural variations in normal hair, the hair growth cycle, and the changes produced by weathering. These structural abnormalities can be congenital or acquired, and some are associated with hair fragility. The classification used here has the advantage of simplicity and is keyed to easy recognition of the abnormalities under the light microscope. The four major types of abnormality are fractures, irregularities, twisting, and extraneous matter affecting the hair shaft. The diagnostic features of the different hair shaft abnormalities are outlined.