Sacropelvic and pelvic fixation constitute the distal foundation of long spinal constructs in adult spinal deformity (ASD), multilevel degenerative disease, high-grade spondylolisthesis, spinopelvic trauma, and complex revision surgery, with the lumbosacral junction exposed to high shear and cantilever forces that make failure at this level a major driver of reoperation and disability . This chapter presents a complication-focused overview of sacropelvic and pelvic instrumentation, addressing indications, mechanisms of failure, prevention, perioperative management, discharge criteria, and structured follow-up, based on a narrative review of biomechanical, clinical, and radiological studies, including recent systematic reviews and large cohort analyses. Modern sacropelvic fixation, particularly S2-alar-iliac (S2AI) –based constructs, has reduced distal junctional failure and rod fracture, yet mechanical complications such as pseudarthrosis, rod fracture, screw loosening, and sacral or pelvic fractures still occur in 5–20% of complex ASD cohorts. S2AI screws show lower rates of hardware prominence and wound problems than conventional iliac screws, with similar or better radiologic correction and reoperation rates . Sacral and pelvic insufficiency fractures, sacroiliac (SI) joint pain, and sacroiliitis are increasingly recognized late complications after long lumbosacral fusion, particularly in older or osteoporotic patients, and pelvic fixation, although associated with greater operative time and blood loss, does not consistently increase systemic perioperative morbidity when enhanced perioperative pathways are applied. Complication-conscious sacropelvic fixation therefore requires meticulous preoperative risk stratification, judicious selection of distal anchors (often favoring S2AI in deformity and degenerative indications), robust lumbosacral fusion strategies (interbody support, multi-rod constructs, adequate graft), and structured postoperative surveillance, with early recognition of characteristic failure patterns and timely, principle-based revision surgery essential to preserving correction and function.