BACKGROUND: Asthma is a chronic inflammatory disease that affects the airways and is common in both adults and children. It is characterised by symptoms including wheeze, shortness of breath, chest tightness, and cough. People with asthma may be helped to manage their condition through shared decision-making (SDM). SDM involves at least two participants (the medical practitioner and the patient) and mutual sharing of information, including the patient's values and preferences, to build consensus about favoured treatment that culminates in an agreed action. Effective self-management is particularly important for people with asthma, and SDM may improve clinical outcomes and quality of life by educating patients and empowering them to be actively involved in their own health. OBJECTIVES: To assess benefits and potential harms of shared decision-making for adults and children with asthma. SEARCH METHODS: We searched the Cochrane Airways Trials Register, which contains studies identified in several sources including CENTRAL, MEDLINE, and Embase. We also searched clinical trials registries and checked the reference lists of included studies. We conducted the most recent searches on 29 November 2016. SELECTION CRITERIA: We included studies of individual or cluster parallel randomised controlled design conducted to compare an SDM intervention for adults and children with asthma versus a control intervention. We included studies available as full-text reports, those published as abstracts only, and unpublished data, and we placed no restrictions on place, date, or language of publication. We included interventions targeting healthcare professionals or patients, their families or care-givers, or both. We included studies that compared the intervention versus usual care or a minimal control intervention, and those that compared an SDM intervention against another active intervention. We excluded studies of interventions that involved multiple components other than the SDM intervention unless the control group also received these interventions. DATA COLLECTION AND ANALYSIS: Two review authors independently screened searches, extracted data from included studies, and assessed risk of bias. Primary outcomes were asthma-related quality of life, patient/parent satisfaction, and medication adherence. Secondary outcomes included exacerbations of asthma, asthma control, acceptability/feasibility from the perspective of healthcare professionals, and all adverse events. We graded and presented evidence in a 'Summary of findings' table.We were unable to pool any of the extracted outcome data owing to clinical and methodological heterogeneity but presented findings in forest plots when possible. We narratively described skewed data. MAIN RESULTS: We included four studies that compared SDM versus control and included a total of 1342 participants. Three studies recruited children with asthma and their care-givers, and one recruited adults with asthma. Three studies took place in the United States, and one in the Netherlands. Trial duration was between 6 and 24 months. One trial delivered the SDM intervention to the medical practitioner, and three trials delivered the SDM intervention directly to the participant. Two paediatric studies involved use of an online portal, followed by face-to-face consultations. One study delivered an SDM intervention or a clinical decision-making intervention through a mixture of face-to-face consultations and telephone calls. The final study randomised paediatric general practice physicians to receive a seminar programme promoting application of SDM principles. All trials were open-label, although one study, which delivered the intervention to physicians, stated that participants were unaware of their physicians' involvement in the trial. We had concerns about selection and attrition bias and selective reporting, and we noted that one study substantially under-recruited participants. The four included studies used different approaches to measure fidelity/intervention adherence and to report study findings.One study involving adults with poorly controlled asthma reported improved quality of life (QOL) for the SDM group compared with the control group, using the Asthma Quality of Life Questionnaire (AQLQ) for assessment (mean difference (MD) 1.90, 95% confidence interval (CI) 1.24 to 2.91), but two other trials did not identify a benefit. Patient/parent satisfaction with the performance of paediatricians was greater in the SDM group in one trial involving children. Medication adherence was better in the SDM group in two studies - one involving adults and one involving children (all medication adherence: MD 0.21, 95% CI 0.11 to 0.31; mean number of controlled medication prescriptions over 26 weeks: 1.1 in the SDM group (n = 26) and 0.7 in the control group (n = 27)). In one study, asthma-related visit rates were lower in the SDM group than in the usual care group (1.0/y vs 1.4/y; P = 0.016), but two other studies did not report a difference in exacerbations nor in prescriptions for short courses of oral steroids. Finally, one study described better odds of reporting no asthma problems in the SDM group than in the usual care group (odds ratio (OR) 1.90, 95% CI 1.26 to 2.87), although two other studies reporting asthma control did not identify a benefit with SDM. We found no information about acceptability of the intervention to the healthcare professional and no information on adverse events. Overall, our confidence in study results ranged from very low to moderate, and we downgraded outcomes owing to risk of bias, imprecision, and indirectness. AUTHORS' CONCLUSIONS: Substantial differences between the four included randomised controlled trials (RCTs) indicate that we cannot provide meaningful overall conclusions. Individual studies demonstrated some benefits of SDM over control, in terms of quality of life; patient and parent satisfaction; adherence to prescribed medication; reduction in asthma-related healthcare visits; and improved asthma control. Our confidence in the findings of these individual studies ranges from moderate to very low, and it is important to note that studies did not measure or report adverse events.Future trials should be adequately powered and of sufficient duration to detect differences in patient-important outcomes such as exacerbations and hospitalisations. Use of core asthma outcomes and validated scales when possible would facilitate future meta-analysis. Studies conducted in lower-income settings and including an economic evaluation would be of interest. Investigators should systematically record adverse events, even if none are anticipated. Studies identified to date have not included adolescents; future trials should consider their inclusion. Measuring and reporting of intervention fidelity is also recommended.