Bone loss occurs during the normal aging process. The term "primary" osteoporosis refers to osteoporosis that results from the involutional losses associated with aging and, in women, additional losses related to natural menopause. Osteoporosis that is caused or exacerbated by other disorders or medication exposures is referred to as "secondary" osteoporosis. This article describes the major causes and provides a framework for the diagnostic investigation of patients with suspected of having secondary osteoporosis.