The diagnostic approach is statistical[1,2]. Age is important: before 5 years old, malignant tumour is almost always metastatic neuroblastoma; between 5 and 15 years old, it is often osteosarcoma or Ewing’s sarcoma; after 40 years old, it tends to be metastasis or myeloma. Clinical symptoms vary little and are most often pain and swelling. Although fever suggests infection, it may also be found in Ewing’s sarcoma. The first step in the evaluation of a tumour is to determine its aggressiveness by conventional radiology. Important parameters include the tumour size, type of matrix, and periosteal reaction. Certain tumours are more common in particular bones. Adamantinoma, usually found in the adult, selectively involves the tibia and fibula. The most common epiphyseal tumour in childhood is the chondroblastoma. In addition, the aggressiveness of some tumours may relate to their location in the axial or appendicular skeleton: in the hand, cartilaginous tumours are almost always benign, whereas in the pelvis, they are often malignant. If necessary, multiple lesions may be estimated with bone scanning. Multiple lesions are seen in chondromas, osteochondromas, histiocytosis X and metastases. The first necessary step is to definitively diagnose benign lesions based on clinical and radiologic signs, and for which biopsy is not necessary. These lesions are fibrous cortical defect, non-ossifying fibroma, periosteal desmoid, fibrous dysplasia, osteochondroma or exostoses, chondroma, simple bone cyst, vertebral angiomas and myositis ossificans. Diagnosis may be difficult. In these cases, the next step is CT. Problems can result from bone locations which are difficult to evaluate on conventional X-ray (short, flat bones especially the pelvis, sacrum, sternum, and vertebrae). Sometimes, the study of the tumour matrix can provide features necessary for the diagnosis. It can be fluid density, small calcifications allowing diagnosis of cartilaginous tumour, osteoid matrix, or fat[3]. CT is the examination of choice in the diagnosis of the nidus of osteoid osteoma in dense bone[4]. Small lytic lesions of the cortex, localized involvement of the soft tissues and thin peripheral periosteal reaction can be seen; lesions with slow evolution which displace and expand the cortex peripherally can be distinguished from more aggressive lesions which cross the cortex. CT can show the tumour on both sides of the cortex before it is destroyed. This is the case for Ewing sarcomas and osteosarcomas[5]. CT allows measurement of the thickness of a non-calcified cuff of a cartilaginous tumour: the cuff is thin in benign lesions and thick (more than 3 cm) in chondrosarcomas[6,7]. In other cases, evidence favours a malignant lesion. Examination for metastases, and MRI examination before biopsy, must be performed.