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The sacro-iliac joint (SIJ) located at the transition between the spine and the lower limbs is subjected to major shear forces. Mobility at the SIJ is very limited but increases during pregnancy and the post-partum period. Familiarity with the anatomy and physiology of the SIJ is important. The SIJ is a diarthrodial joint that connects two variably undulating cartilage surfaces, contains synovial fluid, and is enclosed within a capsule strengthened by several ligaments. This lecture does not discuss rheumatic or inflammatory diseases of the SIJ, whose diagnosis relies on imaging studies and blood tests. Instead, it focuses on micro-traumatic lesions. Micro-trauma causes chronic SIJ pain, which must be differentiated from hip pain and spinal pain. The diagnosis rests on specific clinical provocation tests combined with a local injection of anaesthetic. Findings are normal from radiographs and magnetic resonance imaging. Non-operative treatment with exercise therapy and stretching aims primarily to strengthen the latissimus dorsi, gluteus, and hamstring muscles to increase SIJ coaptation. Other physical treatments have not been proven effective. Radiofrequency denervation of the dorsal sensory rami has shown some measure of efficacy, although the effects tend to wane over time. Patients with refractory pain may benefit from minimally invasive SIJ fusion by trans-articular implantation of screws or plugs, which has provided good success rates.