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This is an update of a Cochrane Review first published in 2001.Hernias are protrusions of all or part of an organ through the body wall that normally contains it. Groin hernias include inguinal (96%) and femoral (4%) hernias, and are often symptomatic with discomfort. They are extremely common, with an estimated lifetime risk in men of 27%. Occasionally they may present as emergencies with complications such as bowel incarceration, obstruction and strangulation. The definitive treatment of all hernias is surgical repair, inguinal hernia repair being one of the most common surgical procedures performed. Mesh (hernioplasty) and the traditional non-mesh repairs (herniorrhaphy) are commonly used, with an increasing preference towards mesh repairs in high-income countries.To evaluate the benefits and harms of different inguinal and femoral hernia repair techniques in adults, specifically comparing closure with mesh versus without mesh. Outcomes include hernia recurrence, complications (including neurovascular or visceral injury, haematoma, seroma, testicular injury, infection, postoperative pain), mortality, duration of operation, postoperative hospital stay and time to return to activities of daily living.We used standard methodological procedures expected by Cochrane. Where available, we collected information on adverse effects. We presented dichotomous data as risk ratios, and where possible we calculated the number needed to treat for an additional beneficial outcome (NNTB). We presented continuous data as mean difference. Analysis of missing data was based on intention-to-treat principles, and we assessed heterogeneity using an evaluation of clinical and methodological diversity, Chi2 test and I2 statistic. We used GRADE to assess the quality of evidence for each outcome.We included 25 studies (6293 participants) in this review. All included studies specified inguinal hernias, and two studies reported that femoral hernias were included.Mesh repair probably reduces the risk of hernia recurrence compared to non-mesh repair (21 studies, 5575 participants; RR 0.46, 95% CI 0.26 to 0.80, I2 = 44%, moderate-quality evidence). In absolute numbers, one hernia recurrence was prevented for every 46 mesh repairs compared with non-mesh repairs. Twenty-four studies (6293 participants) assessed a wide range of complications with varying follow-up times. Neurovascular and visceral injuries were more common in non-mesh repair groups (RR 0.61, 95% CI 0.49 to 0.76, I2 = 0%, NNTB = 22, high-quality evidence). Wound infection was found slightly more commonly in the mesh group (20 studies, 4540 participants; RR 1.29, 95% CI 0.89 to 1.86, I2 = 0%, NNTB = 200, low-quality evidence). Mesh repair reduced the risk of haematoma compared to non-mesh repair (15 studies, 3773 participants; RR 0.88, 95% CI 0.68 to 1.13, I2 = 0%, NNTB = 143, low-quality evidence). Seromas probably occur more frequently with mesh repair than with non-mesh repair (14 studies, 2640 participants; RR 1.63, 95% CI 1.03 to 2.59, I2 = 0%, NNTB = 72, moderate-quality evidence), as does wound swelling (two studies, 388 participants; RR 4.56, 95% CI 1.02 to 20.48, I2 = 33%, NNTB = 72, moderate-quality evidence). The comparative effect on wound dehiscence is uncertain due to wide confidence intervals (two studies, 329 participants; RR 0.55, 95% CI 0.12 to 2.48, I2 = 37% NNTB = 77, low-quality evidence). Testicular complications showed nearly equivocal results; they probably occurred slightly more often in the mesh group however the confidence interval around the effect was wide (14 studies, 3741 participants; RR 1.06, 95% CI 0.63 to 1.76, I2 = 0%, NNTB = 2000, low-quality evidence). Mesh reduced the risk of postoperative urinary retention compared to non-mesh (eight studies, 1539 participants; RR 0.53, 95% CI 0.38 to 0.73, I2 = 56%, NNTB = 16, moderate-quality evidence).Postoperative and chronic pain could not be compared due to variations in measurement methods and follow-up time (low-quality evidence).No deaths occurred during the follow-up periods reported in the seven studies (2546 participants) reporting this outcome (high-quality evidence).The average operating time was longer for non-mesh repairs by a mean of 4 minutes 22 seconds, despite wide variation across the studies regarding size and direction of effect, thus this result is uncertain (20 studies, 4148 participants; 95% CI -6.85 to -1.60, I2= 97%, very low-quality evidence). Hospital stay may be shorter with mesh repair, by 0.6 days (12 studies, 2966 participants; 95% CI -0.86 to -0.34, I2 = 98%, low-quality evidence), and participants undergoing mesh repairs may return to normal activities of daily living a mean of 2.87 days sooner than those with non-mesh repair (10 studies, 3183 participants; 95% CI -4.42 to -1.32, I2 = 96%, low-quality evidence), although the results of both these outcomes are also limited by wide variation in the size and direction of effect across the studies.Mesh and non-mesh repairs are effective surgical approaches in treating hernias, each demonstrating benefits in different areas. Compared to non-mesh repairs, mesh repairs probably reduce the rate of hernia recurrence, and reduce visceral or neurovascular injuries, making mesh repair a common repair approach. Mesh repairs may result in a reduced length of hospital stay and time to return to activities of daily living, but these results are uncertain due to variation in the results of the studies. Non-mesh repair is less likely to cause seroma formation and has been favoured in low-income countries due to low cost and reduced availability of mesh materials. Risk of bias in the included studies was low to moderate and generally handled well by study authors, with attention to details of allocation, blinding, attrition and reporting.