The objective of this study was to evaluate which fusion technique provides the best clinical and radiological outcome for adult low-grade lumbar isthmic spondylolisthesis, and to assess the overall clinical and radiological outcome of each fusion technique. A systematic review was performed. Medline, Embase, Current Contents, and Cochrane databases as well as reference lists of selected articles were searched. Randomised controlled trials (RCTs) were used to evaluate the best treatment; controlled studies and non-controlled studies were used to determine the outcomes after surgery. Two independent reviewers evaluated the studies with the methodological checklists of van Tulder and Jadad for the randomised studies and of Cowley for the non-randomised studies. The search resulted in 684 references and eventually 29 studies met the inclusion criteria, of which eight were RCTs, four were prospective, and 17 were retrospective case series. Ten of the case series did not clearly identify consecutive patient selection. All the eight RCTs evaluated the effect of different techniques of posterolateral fusion (PLF). Evidence was found that the PLF was superior to non-operative treatment (exercise). Circumferential fusion was compared to PLF, but no difference could be found. PLF with or without instrumentation was evaluated in three studies, but no benefits from additional instrumentation were found. Other comparisons within PLF showed no effect of decompression, alternative instrumentation, or bone graft substitute. The 21 case series included 24 patient groups. PLF was used in 15 groups, good or excellent clinical outcome varied from 60 to 98% and fusion rate varied from 81 to 100%. Anterior interbody fusion was used in five groups, good or excellent clinical outcome varied from 85 to 94% and fusion rate varied from 47 to 90%. Posterior interbody fusion was used in two groups, good or excellent clinical outcome was 45% and fusion rate was 80 and 95%, respectively. Reduction, loss of reduction, and lordotic angles before and after the treatment was reported in only four studies. Average reduction achieved was 12.3%, average loss of reduction at follow-up was 5.9%. Preoperative lordotic angles were too heterogeneous to pool the results. Adjacent segment degeneration was not reported in any of the publications. A wide variety of complications were reported in 18 studies and included neurological complications, instrument failure, and infections. Fusion for low-grade isthmic spondylolisthesis has better outcomes than non-operative treatment. The current study could not identify the best surgical technique (PLF, PLIF, ALIF, instrumentation) to perform the fusion. However, instrumentation and/or decompression may play a beneficial role in the modern practice of reduction and fusion for low-grade isthmic spondylolisthesis, but there are no studies yet available to confirm this. The outcomes of fusion are generally good, but reports vary widely.