Osteoclasts are cells of monocyte/macrophage origin that degrade bone matrix. Receptor activator of NF-κB ligand (RANKL) induces osteoclast differentiation in the presence of macrophage colony-stimulating factor. RANKL activates the tumor necrosis factor receptor-associated factor 6, c-Fos, and calcium signaling pathways, all of which are indispensable for the induction and activation of nuclear factor of activated T cells (NFAT) c1. NFATc1 is the master transcription factor for osteoclast differentiation, which regulates many osteoclast-specific genes. Multiple immunoglobulin-like receptors associated with immunoreceptor tyrosine-based activation motif (ITAM)-harboring adapters, Fc receptor common γ subunit (FcRγ), and DNAX-activating protein (DAP) 12 mediate costimulatory signals for RANK, which activate calcium signaling through phospholipase Cγ (PLCγ). In addition to calcineurin-NFATc1, calcium signaling activates the CaMK-CREB (calcium/calmodulin activated kinase-cyclic AMP-response element binding protein) pathway, which also plays a critical role in osteoclastogenesis. This review summarizes recent advances in the study of signaling mechanisms of osteoclast differentiation.