Spinal fusion is frequently performed for a variety of indications. It is performed to treat instability due to trauma, infection, or neoplasm. It may be used to treat regional or global spinal deformity. There are even occasions when it is appropriate as a treatment of low back pain without overt instability or deformity. One common indication for fusion is as an adjunct to decompression for patients with neurogenic claudication or radiculopathy caused by stenosis associated with spondylolisthesis. There have been a number of high-quality publications in high-quality journals that have reported conflicting results regarding the utility of fusion in this patient population. The existence of conflicting data from seemingly similarly designed trials has resulted in some confusion as to when a fusion should be used. This chapter will describe the controversy, discuss the likely basis for the disparate results reported in the literature, and recommend a reasonable treatment strategy. Going forward, the SLIP II study is an ongoing randomized, controlled trial designed to help clarify the situation. Preliminary findings drawn from this study will be discussed.