The osteoclast is the primary bone resorbing cell. It is a highly specialized multinucleated cell whose primary function is to help in the control of calcium homeostasis. The osteoclast has been very difficult to study because of its relative inaccessability, low numbers, and fragility when isolated from bone. Recently, techniques have been developed to study the cell biology of the osteoclast that have expanded our ability to understand the biological and functional properties of osteoclasts. In this article, studies on the origin of the osteoclast are reviewed and the differentiation markers that are used to detect cells in the osteoclast lineage are discussed. Factors that affect osteoclast differentiation are presented and model systems currently in use for studying osteoclast differentiation are evaluated for their relative strengths and weaknesses. In addition, osteoclast differentiation during tooth eruption and root resorption and the effects of bone matrix elements on osteoclast differentiation are reviewed.