The literature on pathologic bone formation is reviewed first on the basis of clinical syndromes and second in relation to newer knowledge of the origin of the bone-forming cells and regulatory molecules. Pathologic bone formation can be categorized into three groups based on the initiating stimulus: trauma, tumors, and idiopathic causes. In the trauma category, the formation of ectopic bone is concerned with major and minor traumatic incidents, surgery, burns, and other causes. In the tumor category, direct and reactive pathologic bone formation is discussed with an emphasis on the different neoplasms capable of ectopic bone formation. The category of idiopathic causes involves the formation of pathologic bone following neurologic injury and in systemic ossification disorders. The origin of the bone-forming cells in all forms of pathologic bone has not been unequivocally determined. However, there is evidence suggesting that these cells may arise from osteogenic stromal elements. Potent bone formation growth-regulating factors have been recently identified, and these molecules must also participate in the formation of pathologic bone. Increased understanding of the processes that control pathologic bone formation will lead to better methods of preventing and treating disorders of ossification.