A basic knowledge of the hair growth cycle is needed to evaluate disorders of hair growth. This chapter presents a broad overview of the physiology and evaluation of hair growth, as well as discussions of specific types of alopecia. The epidemiology, pathogenesis, diagnosis, and treatment of androgenetic alopecia, the most common type of nonscarring hair loss, are covered. Diffuse hair shedding is generalized hair loss over the entire scalp. Diagnosis and treatment of telogen effluvium, anagen arrest (anagen effluvium), and other causes of diffuse hair shedding are covered in detail. Alopecia areata, typically characterized by patchy hair loss; cicatricial alopecia, which results from permanent scarring of the hair follicles; and miscellaneous causes of hair loss are also discussed. Tables list the causes of diffuse and cicatricial alopecia, telogen effluvium, and miscellaneous chemicals and categories of drugs that can cause alopecia, as well as miscellaneous causes of hair loss. Included is an algorithm outlining the approach to diagnosing nonscarring alopecia, as well as a variety of clinical photographs. This review contains 9 highly rendered figures, 6 tables, and 42 references.