Osteoclast-like giant cells (GC) may dominate the histologic pattern not only in conventional giant-cell tumor (GCT)--originating as a radiologically pure lytic, possibly trabeculated lesion especially within the epiphyses of long tubular bones (LB) and pelvic bones of adults--but also in many tumor-like lesions as well as in various benign and malignant bone tumors which may simulate each other. Although the mononuclear cells as well as the amount of collagene fibres don't differ significantly, these lesions can be distinguished by substantial differences concerning their site, radiomorphology and the patients age. The unique lesion which can be recognized by histology only is chondroblastoma--centered in epiphyseal regions as GCT, mostly in the 2nd decade--by its typical mononuclear cells independently of the typical chondroid matrix and calcifications. The brown tumor of hyperparathyreoidism is associated with elevated serum Ca and parathormon, which is not altered in the histologically identical giant cell granuloma. In contrast to GCT aneurysmal bone cyst prefers the metaphyseal area of LB and the posterior parts of vertebras in the 2nd decade. Metaphyseal fibrous defects occurring during growth period leave the epiphyses unaffected and display typical x-rays. Villondoular synovitis sometimes can produce osteolytic defects. GC-rich malignant tumors which present with clear cut atypia except some cases of GC-variants of osteosarcoma, are: Giant-cell rich osteosarcoma, the rare malignant GCT, giant-cell ("osteoclastic") sarcoma, MFH and GC-rich metastases of carcinomas. All of them occur in middle aged and older patients except osteosarcoma and don't affect the epiphyses primarily except malignant GCT. To avoid confusion in GC-lesions it is conditio sine qua non to take into account for diagnosis not only histology but especially radiomorphology as well as site of the lesion and patients age.