Osteopetrosis describes a heterogeneous group of congenital bone disorders characterized by a generalized increase in skeletal mass resulting from a primary reduction in osteoclast mediated bone resorption, The pathogenetic heterogeneity is due to differences in the mutated gene that adversely affect the development or activation of the osteo clast. There are numerous naturally occurring and induced osteopetrotic mutations in animals; like wise, children afflicted with this disease also fall inter various subgroups. The molecular basis for the disease has been determined in some animal mutations, but little is known concerning the etiology of osteopetrosis in most forms in humans. This review will focus on the genetic defects associated with osteopetrosis that have been identified to date and will demonstrate how experimental studies in animal models of osteopetrosis have led to the use of bone marrow transplantation and other alternative strategies to treat osteopetrosis in humans, The clinical outcomes of these therapeutic strategies and potential therapies for use in the future will be discussed. From this review it should be come evident that studies of osteopetrosis in animals will continue to identify the molecular causes for other forms of the disease and to provide information that will help develop new strategies for treatment. In addition, the information gained from these experimental studies is likely to shed additional light on osteoclast biology as well as on broader aspects of skeletal diseases such as inflammation-mediated bone loss and osteoporosis.