Osteoclasts polarise upon activation and form four distinct membrane domains; the basolateral domain, the sealing zone, the functional secretory domain and the ruffled border. The ruffled border is the resorptive organelle of the cell and provides a large surface area for the release of protons and enzymes into the space beneath the osteoclast. Defects in osteoclast formation or function can lead to diseases such as osteopetrosis. Ruffled border formation is a critical event in osteoclast function but the process by which it and other membrane domains form is only partially understood. Vesicular trafficking is essential for the tight regulation of the osteoclast membrane domains and it has been shown previously that treatment with pharmacological inhibitors causes disruption of trafficking. The aims of this PhD were to increase our understanding of vesicular trafficking in osteoclasts and to optimise ways of visualising osteoclast membrane domains. My studies of patients with osteoclast-poor osteopetrosis identified defects in RANKL as a cause of the defect. This in turn has identified a potential therapy of recombinant RANKL for patients with this form of the disease. Although purification of wild type or mutant RANKL was not completely successful, it did suggest that the mutant forms of RANKL were not functional. I have used pharmacological inhibitors to study osteoclast membrane domains, and found that transmission electron microscopy is an essential tool for studying membrane changes following pharmacological inhibition at the ultrastructural level. I also established that the study of vesicular trafficking to analyse formation of membrane domains can make excellent use of immuno-electron methods. Furthermore, genetic diseases associated with defective ruffled border formation such as XLA and osteopetrosis provide useful tools to further analyse the dynamics involved in the formation and maintenance of the ruffled border, as well as revealing more about the diseases themselves.