BACKGROUND: Exercise or physical activity is recommended for improving pain and functional status in people with knee or hip osteoarthritis. These are complex interventions whose effectiveness depends on one or more components that are often poorly identified. It has been suggested that health benefits may be greater with high-intensity rather than low-intensity exercise or physical activity. OBJECTIVES: To determine the benefits and harms of high- versus low-intensity physical activity or exercise programs in people with hip or knee osteoarthritis. SEARCH METHODS: We searched the Cochrane Central Register of Controlled Trials (CENTRAL; issue 06, 2014), MEDLINE (194 8 to June 2014) , EMBASE (198 0 to June 2014), CINAHL (1982 to June 2014), PEDro (1929 to June 2014), SCOPUS (to June 2014) and the World Health Organization (WHO) International Clinical Registry Platform (to June 2014) for articles, without a language restriction. We also handsearched relevant conference proceedings, trials, and reference lists and contacted researchers and experts in the field to identify additional studies. SELECTION CRITERIA: We included randomized controlled trials of people with knee or hip osteoarthritis that compared high- versus low-intensity physical activity or exercise programs between the experimental and control group.High-intensity physical activity or exercise programs training had to refer to an increase in the overall amount of training time (frequency, duration, number of sessions) or the amount of work (strength, number of repetitions) or effort/energy expenditure (exertion, heart rate, effort). DATA COLLECTION AND ANALYSIS: Two review authors independently assessed study eligibility and extracted data on trial details. We contacted authors for additional information if necessary. We assessed the quality of the body of evidence for these outcomes using the GRADE approach. MAIN RESULTS: We included reports for six studies of 656 participants that compared high- and low-intensity exercise programs; five studies exclusively recruited people with symptomatic knee osteoarthritis (620 participants), and one study exclusively recruited people with hip or knee osteoarthritis (36 participants). The majority of the participants were females (70%). No studies evaluated physical activity programs. We found the overall quality of evidence to be low to very low due to concerns about study limitations and imprecision (small number of studies, large confidence intervals) for the major outcomes using the GRADE approach. Most of the studies had an unclear or high risk of bias for several domains, and we judged five of the six studies to be at high risk for performance, detection, and attrition bias.Low-quality evidence indicated reduced pain on a 20-point Western Ontario and McMaster Universities Arthritis Index (WOMAC) pain scale (mean difference (MD) -0.84, 95% confidence interval (CI) -1.63 to -0.04; 4% absolute reduction, 95% CI -8% to 0%; number needed to treat for an additional beneficial outcome (NNTB) 11, 95% CI 14 to 22) and improved physical function on the 68-point WOMAC disability subscale (MD -2.65, 95% CI -5.29 to -0.01; 4% absolute reduction; NNTB 10, 95% CI 8 to 13) immediately at the end of the exercise programs (from 8 to 24 weeks). However, these results are unlikely to be of clinical importance. These small improvements did not continue at longer-term follow-up (up to 40 weeks after the end of the intervention). We are uncertain of the effect on quality of life, as only one study reported this outcome (0 to 200 scale; MD 4.3, 95% CI -6.5 to 15.2; 2% absolute reduction; very low level of evidence).Our subgroup analyses provided uncertain evidence as to whether increased exercise time (duration, number of sessions) and level of resistance (strength or effort) have an impact on the exercise program effects.Three studies reported withdrawals due to adverse events. The number of dropouts was small. Only one study systematically monitored adverse effects, but four studies reported some adverse effects related to knee pain associated with an exercise program. We are uncertain as to whether high intensity increases the number of adverse effects (Peto odds ratio 1.72, 95% CI 0.51 to 5.81; - 2% absolute risk reduction; very low level of evidence). None of the included studies reported serious adverse events. AUTHORS' CONCLUSIONS: We found very low-quality to low-quality evidence for no important clinical benefit of high-intensity compared to low-intensity exercise programs in improving pain and physical function in the short term. There was insufficient evidence to determine the effect of different types of intensity of exercise programs.We are uncertain as to whether higher-intensity exercise programs may induce more harmful effects than those of lower intensity; this must be evaluated by further studies. Withdrawals due to adverse events were poorly monitored and not reported systematically in each group. We downgraded the evidence to low or very low because of the risk of bias, inconsistency, and imprecision.The small number of studies comparing high- and low-intensity exercise programs in osteoarthritis underscores the need for more studies investigating the dose-response relationship in exercise programs. In particular, further studies are needed to establish the minimal intensity of exercise programs needed for clinical effect and the highest intensity patients can tolerate. Larger studies should comply with the Consolidated Standards of Reporting Trials (CONSORT) checklist and systematically report harms data to evaluate the potential impact of highest intensities of exercise programs in people with joint damage.