Hair loss includes excessive shedding of hair (effluvium) and the alopecias, in which there is a decrease in the amount of hair. Both may occur diffusely, or in a circumscribed manner. The localization and pattern of circumscribed hair loss may give a clue to its cause. On the basis of morphologic criteria the alopecias are further classified into non-scarring and scarring types. Non-scarring alopecias (diffuse alopecia, androgenetic alopecia, alopecia areata) are essentially hair cycling disorders, while scarring alopecias result from irreversible destruction of the hair follicles with recognizable loss of follicular osita. Scalp biopsy should be an early step in the evaluation of any case of alopecia, in which scarring is suspected. Too little scalp hair is not a vital problem, but represents a major health care challenge. Only recently the psychological impact of hair loss has been appreciated by the medical community, though throughout history, too little hair has been a concern to mankind, and the object of cosmetic interest and quackery. Recent advances of the medical sciences have led to a better understanding of the underlying pathogenic processes and opened the venue to effective pharmacotherapy (minoxidil, finasteride) and technologies (autologous hair transplantation) for the treatment of the most common disorders. Together with the availability of such treatments, high technical standards for evaluating their efficacy have been developed and have become mandatory for appraisal of any hair growth promoting agent, both in clinical studies and in the individual patient.