Background: Stroke is the third leading cause of disability worldwide. Physical activity is important for secondary stroke prevention and for promoting functional recovery. However, people with stroke are more inactive than healthy age-matched controls. Therefore, interventions to increase activity after stroke are vital to reduce stroke-related disability. Objectives: To summarise the available evidence regarding the effectiveness of commercially available, wearable activity monitors and smartphone applications for increasing physical activity levels in people with stroke. Search methods: We searched the Cochrane Stroke Group Trials Register, CENTRAL, MEDLINE, Embase, CINAHL, SPORTDiscus, and the following clinical trial registers: WHO International Clinical Trials Registry Platform, Clinical Trials, EU Clinical Trial Register, ISRCTN Registry, Australian and New Zealand Clinical Trial Registry, and Stroke Trials Registry to 3 March 2018. We also searched reference lists, Web of Science forward tracking, and Google Scholar, and contacted trial authors to obtain further data if required. We did not restrict the search on language or publication status. Selection criteria: We included all randomised controlled trials (RCTs) and randomised cross-over trials that included use of activity monitors versus no intervention, another type of intervention, or other activity monitor. Participants were aged 18 years or older with a diagnosis of stroke, in hospital or living in the community. Primary outcome measures were steps per day and time in moderate-to-vigorous intensity activity. Secondary outcomes were sedentary time, time spent in light intensity physical activity, walking duration, fatigue, mood, quality of life, community participation and adverse events. We excluded upper limb monitors that only measured upper limb activity. Data collection and analysis: We followed standard Cochrane methodology to analyse and interpret the data. At least two authors independently screened titles and abstracts for inclusion. We resolved disagreements by consulting a third review author. We extracted the following data from included studies into a standardised template: type of study, participant population, study setting, intervention and co-interventions, time-frame, and outcomes. We graded levels of bias as high, low, or unclear, and assessed the quality of evidence for each outcome using the GRADE approach. Main results: We retrieved 28,098 references, from which we identified 29 potential articles. Four RCTs (in 11 reports) met the inclusion criteria.The sample sizes ranged from 27 to 135 (total 245 participants). Time poststroke varied from less than one week (n = 1), to one to three months (n = 2), or a median of 51 months (n = 1). Stroke severity ranged from a median of one to six on the National Institutes of Health Stroke Scale (NIHSS). Three studies were conducted in inpatient rehabilitation, and one was in a university laboratory. All studies compared use of activity monitor plus another intervention (e.g. a walking retraining programme or an inpatient rehabilitation programme) versus the other intervention alone. Three studies reported on the primary outcome of daily step counts. There was no clear effect for the use of activity monitors in conjunction with other interventions on step count in a community setting (mean difference (MD) -1930 steps, 95% confidence interval (CI) -4410 to 550; 1 RCT, 27 participants; very low-quality evidence), or in an inpatient rehabilitation setting (MD 1400 steps, 95% CI -40 to 2840; 2 RCTs, 83 participants; very low-quality evidence). No studies reported the primary outcome moderate-to-vigorous physical activity, but one did report time spent in moderate and vigorous intensity activity separately: this study reported that an activity monitor in addition to usual inpatient rehabilitation increased the time spent on moderate intensity physical activity by 4.4 minutes per day (95% CI 0.28 to 8.52; 1 RCT, 48 participants; low-quality evidence) compared with usual rehabilitation alone, but there was no clear effect for the use of an activity monitor plus usual rehabilitation for increasing time spent in vigorous intensity physical activity compared to usual rehabilitation (MD 2.6 minutes per day, 95% CI -0.8 to 6; 1 RCT, 48 participants; low-quality evidence). The overall risk of bias was low, apart from high-risk for blinding of participants and study personnel. None of the included studies reported any information relating to adverse effects. Authors' conclusions: Only four small RCTs with 274 participants (three in inpatient rehabilitation and one in the community) have examined the efficacy of activity monitors for increasing physical activity after stroke. Although these studies showed activity monitors could be incorporated into practice, there is currently not enough evidence to support the use of activity monitors to increase physical activity after stroke.