With the development of improved diagnostic and treatment options, reduced bone mineral density in children is receiving increased attention. The etiology of osteopenia in healthy children is multifactorial and incompletely understood, but poor calcium intake during the adolescent growth spurt may be an important (and potentially reversible) factor. Other clinically relevant causes of reduced bone mineral density in children include osteogenesis imperfecta, rickets, juvenile rheumatoid and other chronic arthritides, osteopenia associated with neuromuscular disorders, and idiopathic osteoporosis. To provide effective treatment, it is important to understand the process of normal skeletal mineralization, the techniques of bone mineral density measurement, the pathophysiology of osteopenia, and the evaluation and treatment options for the general pediatric population as well as for patients with specific pediatric disorders.