Spondylolisthesis is a forward slippage of one vertebral body over the one below. There are a variety of causes and classification schemes, but most can be described as either degenerative, caused by chronic inter-segmental instability involving degenerative disc and facet joints or isthmic, caused by developmental defects involving the posterior arch of the vertebra. Degenerative spondylolisthesis usually affects older patients, more frequently involves women at the L4/L5 vertebra level, and has relatively small slips (<30%) with associated stenosis. In contrast, isthmic spondylolisthesis usually affects patients younger than 50 years, primarily involves the L5 vertebra level, and may have quite severe progression (>50-100% slip) and associated structural abnormalities, including kyphosis.