BACKGROUND: Inflammatory bowel disease (IBD), comprised of Crohn's disease (CD) and ulcerative colitis (UC), is characterized by chronic mucosal inflammation, frequent hospitalizations, adverse health economics, and compromised quality of life. Diet has been hypothesised to influence IBD activity. OBJECTIVES: To evaluate the efficacy and safety of dietary interventions on IBD outcomes. SEARCH METHODS: We searched the Cochrane IBD Group Specialized Register, CENTRAL, MEDLINE, Embase, Web of Science, Clinicaltrials.gov and the WHO ICTRP from inception to 31 January 2019. We also scanned reference lists of included studies, relevant reviews and guidelines. SELECTION CRITERIA: We included randomized controlled trials (RCTs) that compared the effects of dietary manipulations to other diets in participants with IBD. Studies that exclusively focused on enteral nutrition, oral nutrient supplementation, medical foods, probiotics, and parenteral nutrition were excluded. DATA COLLECTION AND ANALYSIS: Two review authors independently performed study selection, extracted data and assessed bias using the risk of bias tool. We conducted meta-analyses where possible using a random-effects model and calculated the risk ratio (RR) and corresponding 95% confidence interval (CI) for dichotomous outcomes. We assessed the certainty of evidence using GRADE. MAIN RESULTS: The review included 18 RCTs with 1878 participants. The studies assessed different dietary interventions for active CD (six studies), inactive CD (seven studies), active UC (one study) and inactive UC (four studies). Dietary interventions involved either the consumption of low amounts or complete exclusion of one or more food groups known to trigger IBD symptoms. There was limited scope for data pooling as the interventions and control diets were diverse. The studies were mostly inadequately powered. Fourteen studies were rated as high risk of bias. The other studies were rated as unclear risk of bias.The effect of high fiber, low refined carbohydrates, low microparticle diet, low calcium diet, symptoms-guided diet and highly restricted organic diet on clinical remission in active CD is uncertain. At 4 weeks, remission was induced in: 100% (4/4) of participants in the low refined carbohydrates diet group compared to 0% (0/3) of participants in the control group (RR 7.20, 95% CI 0.53 to 97.83; 7 participants; 1 study; very low certainty evidence). At 16 weeks, 44% (23/52) of participants in the low microparticle diet achieved clinical remission compared to 25% (13/51) of control-group participants (RR 3.13, 95% CI 0.22 to 43.84; 103 participants; 2 studies; I² = 73%; very low certainty evidence). Fifty per cent (16/32) of participants in the symptoms-guided diet group achieved clinical remission compared to 0% (0/19) of control group participants (RR 20.00, 95% CI 1.27 to 315.40; 51 participants ; 1 study; very low certainty evidence) (follow-up unclear). At 24 weeks, 50% (4/8) of participants in the highly restricted organic diet achieved clinical remission compared to 50% (5/10) of participants in the control group (RR 1.00, 95% CI 0.39 to 2.53; 18 participants; 1 study; very low certainty evidence). At 16 weeks, 37% (16/43) participants following a low calcium diet achieved clinical remission compared to 30% (12/40) in the control group (RR 1.24, 95% CI 0.67 to 2.29; 83 participants; 1 study; very low certainty evidence).The effect of low refined carbohydrate diets, symptoms-guided diets and low red processed meat diets on relapse in inactive CD is uncertain. At 12 to 24 months, 67% (176/264) of participants in low refined carbohydrate diet relapsed compared to 64% (193/303) in the control group (RR 1.04, 95% CI 0.87 to 1.25; 567 participants; 3 studies; I² = 35%; low certainty evidence). At 6 to 24 months, 48% (24/50) of participants in the symptoms-guided diet group relapsed compared to 83% (40/48) participants in the control diet (RR 0.53, 95% CI 0.28 to 1.01; 98 participants ; 2 studies; I² = 54%; low certainty evidence). At 48 weeks, 66% (63/96) of participants in the low red and processed meat diet group relapsed compared to 63% (75/118) of the control group (RR 1.03, 95% CI 0.85 to 1.26; 214 participants; 1 study; low certainty evidence). At 12 months, 0% (0/16) of participants on an exclusion diet comprised of low disaccharides / grains / saturated fats / red and processed meat experienced clinical relapse compared to 26% (10/38) of participants on a control group (RR 0.11, 95% CI 0.01 to 1.76; 54 participants; 1 study; very low certainty evidence).The effect of a symptoms-guided diet on clinical remission in active UC is uncertain. At six weeks, 36% (4/11) of symptoms-guided diet participants achieved remission compared to 0% (0/10) of usual diet participants (RR 8.25, 95% CI 0.50 to 136.33; 21 participants; 1 study; very low certainty evidence).The effect of the Alberta-based anti-inflammatory diet, the Carrageenan-free diet or milk-free diet on relapse rates in inactive UC is uncertain. At 6 months, 36% (5/14) of participants in the Alberta-based anti-inflammatory diet group relapsed compared to 29% (4/14) of participants in the control group (RR 1.25, 95% CI 0.42 to 3.70; 28 participants; 1 study; very low certainty evidence). Thirty per cent (3/10) of participants following the carrageenan-free diet for 12 months relapsed compared to 60% (3/5) of the participants in the control group (RR 0.50, 95% CI 0.15 to 1.64; 15 participants; 1 study; very low certainty evidence). At 12 months, 59% (23/39) of milk free diet participants relapsed compared to 68% (26/38) of control diet participants (RR 0.83, 95% CI 0.60 to 1.15; 77 participants; 2 studies; I² = 0%; low certainty evidence).None of the included studies reported on diet-related adverse events. AUTHORS' CONCLUSIONS: The effects of dietary interventions on CD and UC are uncertain. Thus no firm conclusions regarding the benefits and harms of dietary interventions in CD and UC can be drawn. There is need for consensus on the composition of dietary interventions in IBD and more RCTs are required to evaluate these interventions. Currently, there are at least five ongoing studies (estimated enrollment of 498 participants). This review will be updated when the results of these studies are available.