Magnetic resonance imaging (MRI) provides a non-invasive means to evaluate a large fraction of marrow in less than one hour. Marrow disorders produce non-specific changes in marrow signal intensities which primarily reflect changes in proportions of fat and cellular elements. The pattern of these signal changes narrows the differential diagnosis, and the combination of these features with the clinical context allows interpretations which are clinically useful in many ways. These include: 1) the diagnosis of avascular necrosis (and its distinction from other causes of joint pain), 2) detection of osteomyelitis, 3) differential diagnosis of hypoplastic disorders, 4) staging of lymphomas and myeloma, 5) selection of patients for autologous bone marrow transplant, 6) objective measures of marrow response to therapy, 7) detection of leukemic transformation, and 8) improved detection of marrow disease (primary or secondary) in patients with otherwise unexplained bone pain.