OBJECTIVE: The objective of this systematic review was to determine the effect of lumbar spine-strengthening exercises on outcomes for people with chronic low back pain. METHODS: Two independent reviewers followed Cochrane Back Review Group and QUORUM Statement guidelines to complete this systematic review. Exercise effects were reported as standardized mean difference (SMD) with 95% confidence intervals. RESULTS: Thirteen high-quality randomized controlled trials were included. For chronic low back pain, trunk strengthening is more effective than no exercise on long-term pain (SMD 0.95 [0.35-1.55]; intensive trunk strengthening is more effective than less intensive on function (pooled SMD: short-term, 0.58 [0.22-0.94]; long-term, 0.77 [0.33-1.20]). Compared with physiotherapy or aerobics, effects are comparable on pain and function. Motivation strategies increase effectiveness. After disk surgery, effects are significant for function (pooled SMD: short-term, 1.08 (0.76-1.41); long-term, 0.53 (0.03-1.04). For severe degeneration, trunk strengthening is less favorable than fusion on long-term pain (SMD, -0.50 [-0.99 to -0.01]) or function (SMD, -0.76 [-1.25 to -0.26]). Intensive trunk strengthening is less effective than McKenzie exercises for pain reduction (SMD: short-term, -0.29 [-0.54 to -0.05]; long-term, -0.31 [-0.55 to -0.06]). We estimated that moderate effect sizes (0.5) indicate that approximately 50% of participants and large effect sizes (0.8) indicate that approximately 80% of participants would achieve important improvement. CONCLUSIONS: Trunk strengthening appears effective compared with no exercise. Increasing exercise intensity and adding motivation increase treatment effects. Trunk strengthening, compared with aerobics or McKenzie exercises, showed no clear benefit of strengthening. It is unclear whether observed benefits are due to tissue loading or movement repetition.