Much of the morbidity and mortality associated with the plasma cell (PC) malignancy, multiple myeloma (MM), is owing to the severe osteolytic bone disease seen in patients with this disease. Although the molecular mechanisms responsible for osteolysis remain to be fully elucidated, it is clear from numerous studies that it is owing, in part, to an increase in osteoclastic bone resorption. Several known osteoclast (OC)-activating factors (OAFs) are produced by myeloma PCs (MPCs), or by stromal cells in response to MPCs and include interleukin-1beta (IL-1beta); tumor necrosis factor-alpha (TNF-alpha); IL-6; parathyroid hormone-related protein; macrophage inflammatory protein-1alpha; and, most recently, the TNF-ligand family member receptor activator of nuclear factor-kappaB ligand (RANKL). The identification and significance of any one of these myeloma-derived OAFs is dependent on robust and reliable assays that measure the de novo formation and activation of OCs. A number of in vitro assay systems have been described that examine the requirements for normal OC formation and are easily adaptable for examining which MM-derived OAF and to what extent it is responsible for the bone loss observed in individuals with myeloma. This chapter describes one such in vitro model system.