Bone is a specialized connective tissue that performs many important functions: (i) mechanical, supporting the whole body and allowing the movements; (ii) protective, shielding many vital organs, such as brain, lung, heart and bone marrow; (iii) metabolic, regulating the homeostasis of calcium and phosphate (Baron, 1999); (iv) endocrine, regulating kidney function (Fukumoto & Martin, 2009; Mazzaferro et al., 2010) and contributing to global energy balance (Ducy et al., 1996; Ferron et al., 2010; Lee et al., 2007) and male fertility (Oury et al., 2011). Bone is a dynamic tissue, subjected to a continuous process of renewal and remodelling in which bone resorption by osteoclasts and bone formation by osteoblasts occur at the same site along the bone surface (Pogoda et al., 2005). About 10% of bone is replaced each year, with complete skeletal renewal every 10 years. An imbalance between osteoblast and osteoclast activities can cause serious consequences: if bone formation is enhanced or bone resorption is impaired, bone mass is increased, and vice versa (Parfitt, 1982; Pogoda et al., 2005). Often osteoclast diseases are monogenic, and in many of them the responsible gene and the respective function have been identified, while for other osteoclast diseases the causative gene has not been isolated or the exact function of the matching protein still remains unknown. In this review, a brief description of osteoclast biology will be provided and examples of genetic osteoclast diseases, including osteopetrosis, pycnodysostosis and Paget’s disease of bone, will be discussed.