Hair loss is a common and distressing symptom. However, research on the hair biology had been relatively lacking until several decades ago. Recently a lot of studies on hair were conducted and experimental works uncovered a secret of the hair growth. The hair follicle is a highly organized structure with its unique growth cycle. This review focuses on the biology of hair growth and diagnostic methods of hair loss. Also, etiopathogenesis and treatment of alopecia areata and androgenetic alopecia is discussed.