The iliopsoas compartment is an extraperitoneal space that contains the greater psoas, smaller psoas, and iliac muscles. Many pathologic processes may involve the iliopsoas compartment, including inflammatory, hemorrhagic, and neoplastic conditions. Psoas muscle infection is usually due to direct extension from contiguous structures. With the decreasing incidence of tuberculosis, the majority of psoas abscesses now encountered have a pyogenic origin. Hemorrhage into the psoas muscle can be spontaneous or secondary to various conditions. Neoplastic involvement of the psoas muscle is usually due to contiguous spread and is rarely primary. With the refinement of imaging modalities, there has been increased recognition of diseases that involve the iliopsoas compartment. Although these conditions may look similar radiologically, they can be correctly diagnosed by combining the radiologic findings with the clinical history. Biopsy is effective in diagnosis of such conditions; aspiration and drainage are effective in both diagnosis and therapy.