Myositis ossificans (ossifying myositis) is a form of ectopic ossification, first described by Guy Patio in 1692. According to the WHO definition, myositis ossificans is a non-neoplastic lesion characterized by the proliferation of fibrous tissue with extensive new bone formation, and there can be cartilage formation. Osteomyositis often occurs after trauma, in the ventral muscles of the shoulder, elbow, hip, thigh and calf, often in the bone-muscle junction, but also in the muscle bonds, fascia, periosteum, vascular wall, and ligaments. It can range from small with no clinically significant ossification symptoms to large with widespread heterotopic ossification throughout the body, manifesting as varying degrees of motor dysfunction. This sequence of articles describes the concept, pathogenesis, ancillary tests, diagnosis and differential diagnosis, and treatment of limited ossifying myositis as an example.