Bone homeostasis depends on balanced bone deposition and bone resorption, which are mediated by osteoblasts and osteoclasts, respectively. The process of bone turnover requires the coordination of these cells. Changes in the ability of either cell type to perform its function results in pathological conditions such as osteoporosis and tumor-induced bone loss (osteolysis). The number of osteoclasts present at the site of bone remodeling as well as the activity of those osteoclasts the control amount of bone resorbed (1). Therefore, factors affecting overall numbers of osteoclasts and osteoclast activation are key to regulating bone loss. Osteoclast numbers are in part controlled by osteoclast differentiation from bone marrow precursors of the monocyte/macrophage lineage (2). Differentiation of these hematopoietic precursors into osteoclasts is supported by bone marrow stromal cell production of two cytokines, receptor activator of NF-κB ligand (RANKL) and macrophage colony stimulating factor (M-CSF), which are both necessary and sufficient to mediate osteoclast differentiation (3, 4). Although RANKL production by the stroma supports osteoclast differentiation, this process is antagonized by osteoprotogerin (OPG) production, which acts as a soluble decoy receptor for RANKL (5, 6). Mechanistic studies to elucidate the factors influencing bone metabolism necessitate in vitro studies of osteoclast differentiation, activation and survival. There are a number of in vitro methods used to culture and study osteoclasts, some of which are described in this chapter.