Metastatic cancer often involves the skeleton. Tumor most often reaches the bones by hematogenous spread; however, direct extension from the primary tumor or from another site of metastasis, as well as lymphatic dissemination, may occur. Clinical features depend on the affected sites and the extent of disease. The radioisotope bone scan has been the preferred imaging screening modality. Magnetic resonance imaging is probably more sensitive in the detection of axial lesions, but additional development is needed before it can replace the isotope scan in evaluation of the long bones. For patients presenting with metastatic disease, the appearance of the lesions may help to guide the search for a primary. Some helpful patterns include lytic and sclerotic lesions, "expanded" lesions, "pseudosarcomatous" lesions, acral lesions, and soft tissue ossification. Evaluation of response to therapy is problematic. Progressive sclerosis of a lytic focus generally indicates a positive response. Pitfalls in the evaluation of response include the "flare" phenomenon, which has been observed on radioisotope scans early in the course of therapy.