1) Spondylolisthesis is the slippage of one vertebra over another, usually occurring at the L5-S1 level. It can be asymptomatic but sometimes causes back and leg pain. 2) The causes include dysplasia of the pars interarticularis, fractures from repetitive stress, and activities that increase lumbar lordosis. High grade spondylolisthesis affects over 50% of the vertebral body. 3) Sagittal balance and alignment of the pelvis and spine are important factors in determining treatment. Patients with altered sagittal balance or pelvic parameters may be more prone to spondylolisthesis due to increased mechanical stress at L5-S1.