Systematic review. This systematic review compares radiographic and clinical outcomes between instrumented and noninstrumented posterolateral lumbar spine fusions for the treatment of degenerative lumbar spondylolisthesis. The optimal method of fusion for instability from degenerative lumbar spondylolisthesis remains to be an area of debate amongst spine surgeons. There are no prior comprehensive systematic review of comparative studies that compares outcomes between instrumented and noninstrumented posterolateral spine fusions for the treatment of degenerative lumbar spondylolisthesis. A systematic review was registered with PROSPERO and performed according to Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines using the PubMed, SCOPUS, and Ovid MEDLINE databases. All level I-III comparative studies published in the English language investigating the clinical outcomes between instrumented and noninstrumented posterolateral spine fusions for the treatment of degenerative lumbar spondylolisthesis were included. Seven studies (672 patients, 274 noninstrumented, 398 instrumented) were analyzed. One randomized study was level I evidence, 2 randomized studies were level II, and 4 nonrandomized studies were level III. Mean follow-up ranged from 1.4 to 5.9 years. Instrumented patients had a higher rate of solid fusion (87.6% vs. 77.1%, P=0.023) and a lower rate of definitive pseudarthrosis (5.3% vs. 19.9%, P<0.001). However, there was no difference in overall functional improvement at final follow-up between the 2 treatment groups (75.0% vs. 81.7%, P=0.258). In addition, there was no difference in reoperation or complication rates. For the treatment of degenerative lumbar spondylolisthesis, there are significantly higher rates of fusion among patients undergoing instrumented posterolateral fusion compared with noninstrumented posterolateral fusion. However, there is no difference in overall functional improvement, pain-related outcome scores, reoperation rates, or complication rates between the 2 treatment groups. Level III-systematic review of level I-III studies.