Rickets, osteomalacia, and renal osteodystrophy are disorders of the mineralization of bone that result from a lack of available calcium, phosphorus, or both. The diseases that result from numerous mechanisms present with a symptom-sign-radiographic complex with such a high degree of stereotypy that laboratory investigation is often required to distinguish one form from another. The disorders in children, known as rickets, produce bowing and other deformities of the long bones and dwarfism. These disorders are principally related to the profound effect of the deficiency states on the epiphyseal plate; whereas the same disorders in adults produce an often severe osteopenia and pathologic fractures. Because of newer developments in our understanding of the factors affecting calcium-phosphorus hemostasis and vitamin D metabolism, many of the children and adults with these disorders can now be treated effectively.