Bisphosphonates are synthetic analogues of pyrophosphate. Their main pharmacological effect is to iuhibit bone resorption by a variety of mechanisms, not all of which are clearly understood. The activity of the bisphosphonates varies depending on the compound. In clinical trials, they have been shown to stop postmenopausal bone loss and increase bone density, with a concomitant reduction in fracture rate with some agents. This article reviews the currently known mechanisms of action of the bisphosphonates and the evidence that they are useful in the treatment of osteoporosis.