Cartilage lesions of bone are the most difficult ones for the pathologist to classify. The reasons for this are discussed. The borderline cartilage lesions include osteochondroma, enchondroma, chondrosarcoma, chondroblastoma, chondromyxoid fibroma, juxtacortical chondroma and chondrosarcoma, clear cell chondrosarcoma, and the peripheral chondrosarcoma. Specific differential points are provided with illustrative material and a summary table in order to ease the pathologist's task.