Hair pigmentation and graying are important topics for the understanding of the physiology of aging; the differentiation of stem cells; and the mechanisms underlying disease processes such as progeroid syndromes, vitiligo, and hypothyroidism. Although hair graying, or canities, is a common process occurring in people as they age, an unknown percentage of individuals experience premature graying from familial inheritance or pathologic conditions. We review the physiology of hair pigmentation and the mechanism underlying physiologic graying, and we explore the etiology of pathologic causes of premature graying, pathologies associated with premature graying, and the limited available treatment options for hair graying.