Bone mass is maintained by two specialised types of cells: bone-forming osteoblasts and bone-resorbing osteoclasts. Osteoclasts are tartrate-resistant acid phosphatase-positive, multinucleated giant cells that originate from the monocyte/macrophage lineage of hematopoietic stem cells. Chapter one of this book describes the role miRNAs have in osteoclast differentiation and function. Chapter two investigates how the inhibition of complement activity influenced joint destruction in relation to osteoclast differentiation. Chapter three is a discussion on the effect of sepsis and bacterial infection on immune system activation, bone remodeling and osteoclast activation. The final chapter of the book provides a current view of possible roles of DC-STAMP in signalling.