The sacroiliac (SI) joint is a complex joint that allows energy transfer between the torso and the lower extremities. The sacroiliac is a true synovial joint with proven motion.¹,² The wedged shape of the sacrum and the lack of interdigitation of the ilium leads to the SI joint relying on force closure of the joint rather than form closure.³, ⁴, ⁵ This is supported by the increased rate of friction within the SI joint. The SI joint is highly innervated both dorsally and ventrally⁶, ⁷, ⁸ with pain reproduced with noxious stimuli⁹,¹⁰ and relieved with anesthetic injection.¹¹, ¹², ¹³ The reported prevalence of SI joint pain ranges from 15% to 30% as a component of low back pain and 32% to 43% as a component of symptomatic post-lumbar fusion pain.¹⁴, ¹⁵, ¹⁶, ¹⁷ About 75% of post-lumbar spine fusion patients having radiographic evidence of SI joint degeneration¹⁸ and 32% having definitive SI joint pain following lumbar fusion.¹⁴ The SI joint presents a unique diagnostic challenge as referral pain patterns from the spine, SI joints, and hips overlap. Diagnosis includes a positive history,¹⁹ positive exam findings with point localization over the SI joint,²⁰ and positive provocative tests (distraction, thigh thrust, compression, FABER, Gaenslen’s).¹² After discovering clinical evidence of SI joint pain, a diagnostic injection is key to gaining a definitive diagnosis.¹⁷,²¹,²² If nonoperative management fails to improve symptoms, definitive surgical management includes fusing the SI joint, which in general involves placing fixation across the joint either anteriorly or posteriorly. This chapter aims to demonstrate that minimally invasive surgical (MIS) fusion of the SI joint is safe and effective and can be achieved with decreased morbidity currently compared with traditional open fusion techniques.