In conclusion, the attempt has been made to demonstrate that three major forms of new bone formation exist: reactive, neoplastic, and the newborn or relative skeletal sclerosis in congenital (developmental) disorders. A classification of skeletal disorders has been presented and four major groups have been selected from the nine categories in this classification. These are: congenital-developmental, metabolic and endocrine, benign neoplasms and malignant neoplasms. In all four categories a large group of entities which may present with new bone (sclerosis) are listed and are discussed in some, but limited, detail. A number of these entities in each of the four categories are illustrated. Some difficulty is encountered in considering the mechanisms for the production of bony sclerosis in the group of congenital-developmental disorders. In such entities as osteopetrosis, the overproduction of cartilage cords and subsequent excessive mineralization is known to be responsible for the dense bone. However, in various skeletal dysplasias (e.g., pyknodysostosis, van Buchem disease), the exact mechanism for the development of the diffuse sclerotic process is not clearly understood. In the metabolic and endocrine category, the situation as to mechanism is less unclear in considering the reason for the development of bony sclerosis. Yet even in evaluating disorders such as renal osteodystrophy, the reactive bony sclerosis in the presence of secondary hyperparathyroidism and osteomalacia is a source of speculation with no definite proof, as yet.