Osseous tumors may arise from the periosteum or from the marrow. If they arise from the periosteum they may extend early to the adjacent soft tissues and involve and destroy them. If the tumor arises in the marrow, it is for a long while cut off from the adjacent soft tissues by the thick cortex, and about the extending medullary tumor may also come a reactive proliferation by the periosteum, so that as the tumor extends it still may, for a long time, be surrounded by a shell of bone which prevents infection of the soft parts. After a time, however, the reactive periosteum shell usually becomes perforated at one or more points, and then the medullary tumor extends to the adjacent tissues. The cause of these tumors is absolutely unknown. +Fibromata+ are not very common tumors of bone. They arise generally from the periosteum and are most common about the face, and are rarely seen in the long bones. Many of these tumors are closely allied to some of the fibrous forms of sarcoma, and it is often difficult to distinguish them histologically. +Chondromata+ are fairly common tumors of bone. They may appear externally to the cortex, or sometimes they grow in the medullary canal. They may arise directly from the marrow, probably from remnants of the provisional cartilage cells. They also appear frequently to arise from the epiphyseal line. Chondromata appear generally as multiple masses, nodular in shape, and are frequently seen on the lower leg, about the knee joint. They usually are painless, firm and hard, and not tender to pressure. +Treatment+ consists in removal by operation. +Osteomata+ are bony tumors which generally arise by growth of the periosteum, and form solid bony masses external to the cortex of the bone, when they are called _exostoses_. The density of the bone composing the tumor varies a great deal, some being very hard and ivorylike, while others are like the cellular marrow of the long bones. Osteomata may be surrounded by a layer of fibrous periosteum or, in certain cases, beneath the periosteum appears a layer of cartilage producing the so-called _exostosis cartilaginea_. The latter formation is the one which is most common in the vicinity of the epiphyseal line of the long bones, notably of the leg. Osteomata form circumscribed hard nodular masses of bony consistency, and are usually painless. They may cause interference with function from their size, especially when they appear in close connection with a joint. +Treatment+ is complete and thorough removal. +Sarcomata+ are the most common tumors of bone; they are malignant, and when removed, tend to recur, either locally or by metastasis, in different parts of the body. The metastases usually are distributed by the circulation.