Bone loss is a common finding in the spondyloarthropathies. It may be localized and present as erosions or be generalized and cause osteoporosis. The pathogenesis of bone loss in the spondyloarthropathies is yet to be fully understood. There is however increasing evidence to support a role for the osteoclasts in bone erosions. Similarly a balance between the receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) levels is thought to regulate osteoclastic activity and therefore bone loss in the inflammatory arthritides. In this chapter we will review the recent literature on the role of osteoclasts and the RANKL/OPG system in the various spondyloarthropathies and try to formulate a hypothesis for the possible mechanism of bone loss in this group of inflammatory rheumatic disease.