The immune and skeletal systems are closely related through a number of shared regulatory molecules including cytokines. Studies on bone destruction associated rheumatoid arthritis (RA) as well as identification of the various bone phenotypes found in immune-compromised genetically modified mice have highlighted the importance of the interplay between the two systems, and promoted the new interdisciplinary field of "osteoimmunology" . Accumulating evidence has indicated that bone destruction associated with RA is caused by the enhanced activity of osteoclasts, resulting from the activation of a unique helper T cell subset, "Th17 cells" . The osteoimmunological insight is of growing importance in clinical applications. Furthermore, recent studies has suggested the relationship between bone cells and hematopoietic stem cells in bone marrow. Various cell types in bone marrow are expected to control bone homeostasis, calcium metabolism and hematopoiesis by mutually affecting each other. Osteoimmunology becomes the viewpoint indispensable for not only bone and mineral research but also immunological research.