A background to metabolic bone disease with a focus on Osteoporosis. Properties of trabecular bone. Properties of cortical bone. Remodelling enables bone to respond to mechanical and metabolic demands. RANK Ligand is the primary mediator of bone resportion. RANK Ligand is required for osteoclast formation, function and survival. OPG (Osteoprotegerin) is the endogenous inhibitor of the action of RANK Ligand (decoy receptor). The RANK Ligand : OPG ratio is a critical determinant of BMD.