The osteopetroses are a heterogeneous group of bone remodeling disorders characterized by an increase in bone density due to a defect in osteoclastic bone resorption. In humans, several types can be distinguished and a classification has been made based on their mode of inheritance, age of onset, severity, and associated clinical symptoms. The best-known forms of osteopetrosis are the malignant and intermediate autosomal recessive forms and the milder autosomal dominant subtypes. In addition to these forms, a restricted number of cases have been reported in which additional clinical features unrelated to the increased bone mass occur. During the last years, molecular genetic studies have resulted in the identification of several disease-causing gene mutations. Thus far, all genes associated with a human osteopetrosis encode proteins that participate in the functioning of the differentiated osteoclast. This contributed substantially to the understanding of osteoclast functioning and the pathogenesis of the human osteopetroses and will provide deeper insights into the molecular pathways involved in other bone pathologies, including osteoporosis.