In recent years, substantial progress has been made in understanding the mechanism for bisphosphonate suppression of bone turnover. Bisphosphonates can now be distinguished based on their molecular and cellular mechanisms of action. Simple bisphosphonates such as clodronate and etidronate inhibit bone resorption through induction of osteoclast apoptosis. Clodronate, and perhaps etidronate, triggers apoptosis by generating a toxic analog of adenosine triphosphate, which then targets the mitochondria, the energy center within the cell. For nitrogen-containing bisphosphonates, the direct intracellular target is the enzyme farnesyl diphosphate synthase in the cholesterol biosynthetic pathway. Its inhibition suppresses a process called protein geranylgeranylation, which is essential for the basic cellular processes required for osteoclastic bone resorption. Although nitrogen-containing bisphosphonates can induce osteoclast apoptosis, this is not necessary for their inhibition of bone resorption.