Although bone marrow examination is a common procedure in the evaluation of patients with cancer, its role and contribution have been questioned in recent years. This review deals with the clinical and biologic aspects of metastasis to the bone marrow. The discussion is focused on the common tumor types that involve marrow and the application of newer techniques for tumor detection in the marrow. Therapeutic and prognostic implications of bone marrow metastasis are significant in several clinical settings. The mechanisms by which tumor cells affect marrow function have not been completely defined.