Osteoimmunology is an interdisciplinary research field that investigates the interplay of the skeletal and immune systems at the molecular level. The interaction between immune cells and osteoclasts has been a major topic in this field. As osteoclasts are derived from the monocyte/macrophage lineage, the macrophage-osteoclast interaction has long been studied. The T cell-osteoclast interaction has also attracted much attention in the study of bone destruction in arthritis. In addition, recent reports have revealed new players linking osteoclasts and other immune cells, including B cells and dendritic cells. Thus, a large number of molecules are in fact shared by osteoclasts and immune cells. These findings will lead to a better understanding of the pathogenesis of diseases affecting both systems and will provide a molecular basis for novel therapeutic strategies.