The diagnosis of primary malignant bone tumors, which only account for 1% of malignancies in humans, represents quite a challenge for the pathologist. Apart from the often overlapping morphology between different entities, heterogeneity within one and the same tumor has to be kept in mind. Furthermore, in most cases the diagnosis must be established based on small biopsy specimens and occasionally even on frozen sections. In this context detailed clinical information including the patient's age and exact localization of the tumor, as depicted by X-ray or MRI, are essential requirements for a correct diagnosis. In addition, multiple biopsies from different tumor sites may be of help. For example, 25% of osteogenic sarcomas (OSA) are chondroplastic. Others may contain areas similar to fibrosarcoma (FSA) or malignant fibrous histiocytoma (MFH). Since 90% of OSA occur in the second decade of life whereas chondrosarcomas, FSA and MFH usually manifest after the age of 40 years, a biopsy solely composed of malignant chondromatous tissue obtained from a tibial lesion of a 10-year-old child should most likely be considered as part of an OSA. In the differential diagnosis of the so-called small, round cell tumors, which include Ewing sarcoma, malignant lymphoma and small-cell OSA, immunohistochemical staining of the paraffin-embedded material and a molecular biological/genetic work-up of frozen material are most helpful. In this paper the histomorphological characteristics and criteria for differential diagnosis of the most common primary malignant bone tumors are discussed.