In this article the authors discuss the syndrome of isthmic spondylolisthesis occurring at the lumbosacral junction in adults, providing a description of the clinical syndrome, altered biomechanics, and imaging characteristics. The authors pose arguments in favor of reduction and instrument-assisted fusion. Their surgical technique is detailed. They describe transsacral interbody fusion in which fibular allograft and in situ fixation are used, which they consider the second-best alternative technique when attempts at reduction fail.