Obesity increases the risk of many serious illnesses such as coronary heart disease, type 2 diabetes and osteoarthritis. More men than women are overweight or obese in the UK but men are less likely to perceive their weight as a problem and less likely to engage with weight-loss services.The aim of this study was to systematically review evidence-based management strategies for treating obesity in men and investigate how to engage men in obesity services by integrating the quantitative, qualitative and health economic evidence base.Electronic databases including MEDLINE, EMBASE, PsycINFO, the Cochrane Central Register of Controlled Trials, the Database of Abstracts of Reviews of Effects and the NHS Economic Evaluation Database were searched from inception to January 2012, with a limited update search in July 2012. Subject-specific websites, reference lists and professional health-care and commercial organisations were also consulted.Six systematic reviews were conducted to consider the clinical effectiveness, cost-effectiveness and qualitative evidence on interventions for treating obesity in men, and men in contrast to women, and the effectiveness of interventions to engage men in their weight reduction. Randomised controlled trials (RCTs) with follow-up data of at least 1 year, or any study design and length of follow-up for UK studies, were included. Qualitative and mixed-method studies linked to RCTs and non-randomised intervention studies, and UK-based, men-only qualitative studies not linked to interventions were included. One reviewer extracted data from the included studies and a second reviewer checked data for omissions or inaccuracies. Two reviewers carried out quality assessment. We undertook meta-analysis of quantitative data and a realist approach to integrating the qualitative and quantitative evidence synthesis.From a total of 12,764 titles reviewed, 33 RCTs with 12 linked reports, 24 non-randomised reports, five economic evaluations with two linked reports, and 22 qualitative studies were included. Men were more likely than women to benefit if physical activity was part of a weight-loss programme. Reducing diets tended to produce more favourable weight loss than physical activity alone (mean weight change after 1 year from a reducing diet compared with an exercise programme -3.2 kg, 95% CI -4.8 kg to -1.6 kg). The type of reducing diet did not affect long-term weight loss. A reducing diet plus physical activity and behaviour change gave the most effective results. Low-fat reducing diets, some with meal replacements, combined with physical activity and behaviour change training gave the most effective long-term weight change in men [-5.2 kg (standard error 0.2 kg) after 4 years]. Such trials may prevent type 2 diabetes in men and improve erectile dysfunction. Although fewer men joined weight-loss programmes, once recruited they were less likely to drop out than women (difference 11%, 95% CI 8% to 14%). The perception of having a health problem (e.g. being defined as obese by a health professional), the impact of weight loss on health problems and desire to improve personal appearance without looking too thin were motivators for weight loss amongst men. The key components differ from those found for women, with men preferring more factual information on how to lose weight and more emphasis on physical activity programmes. Interventions delivered in social settings were preferred to those delivered in health-care settings. Group-based programmes showed benefits by facilitating support for men with similar health problems, and some individual tailoring of advice assisted weight loss in some studies. Generally, men preferred interventions that were individualised, fact-based and flexible, which used business-like language and which included simple to understand information. Preferences for men-only versus mixed-sex weight-loss group programmes were divided. In terms of context, programmes which were cited in a sporting context where participants have a strong sense of affiliation showed low drop out rates and high satisfaction. Although some men preferred weight-loss programmes delivered in an NHS context, the evidence comparing NHS and commercial programmes for men was unclear. The effect of family and friends on participants in weight-loss programmes was inconsistent in the evidence reviewed - benefits were shown in some cases, but the social role of food in maintaining relationships may also act as a barrier to weight loss. Evidence on the economics of managing obesity in men was limited and heterogeneous.The main limitations were the limited quantity and quality of the evidence base and narrow outcome reporting, particularly for men from disadvantaged and minority groups. Few of the studies were undertaken in the UK.Weight reduction for men is best achieved and maintained with the combination of a reducing diet, physical activity advice or a physical activity programme, and behaviour change techniques. Tailoring interventions and settings for men may enhance effectiveness, though further research is needed to better understand the influence of context and content. Future studies should include cost-effectiveness analyses in the UK setting.This project was funded by the NIHR Health Technology Assessment programme.