I. Introduction BONE is a dynamic tissue that constantly undergoes remodeling. Bone remodeling is a coupled process in which bone resorption is normally followed by new bone formation. During early life, bone formation exceeds bone resorption with a net increase in bone mass, while late in life, bone resorption exceeds bone formation with net loss of bone. During some pathological processes, such as in patients with advanced stages of multiple myeloma, bone remodeling is uncoupled, and bone resorption is not followed by new bone formation. The primary cell responsible for bone resorption is the multinucleated osteoclast. Although many questions still remain unanswered about the factors that regulate osteoclast formation and osteoclastic bone resorption, major advances have been made recently in our understanding of the cell biology and molecular biology of the osteoclast, as well as the role that the marrow microenvironment plays in regulating osteoclast formation and bone resorption.