Sacroiliitis is a common cause of low back pain with widely varying prevalence. It is defined as inflammation of one or both of the sacroiliac (SI) joints. The size, shape, and daily mechanical loads of the SI joint all contribute to the development of sacroiliitis. The joint is a relatively small space with minimal range of motion and a large ligamentous complex. Varying innervation of the joint can lead to differing referred pain patterns among individuals. This adds complexity to an already difficult diagnosis. Although commonly a result of osteoarthritis, it is important for the reader to understand that the SI joint is a target of autoimmune processes, such as seronegative spondyloarthropathies, mechanical load shifting and hormone release during pregnancy, and spinal fusion surgery. Diagnosis of the disorder is multimodal and includes a physical exam (with provocative testing), imaging, and serum testing. Treatment can range from conservative management to surgery. As sacroiliitis is becoming a more recognized condition, more treatment options can be offered to patients. After completing this chapter, the reader should have a clear understanding of the etiologies, pathophysiology, diagnosis, and management of sacroiliitis.