IntroductionRecruitment and retention of participants in randomized controlled trials (RCTs) is challenging, and is why many RCTs fail or are not completed on time. Digital approaches such as social media, data mining, email or text messaging could improve recruitment and/or retention, but how well they match these purposes is unclear. We used systematic methods to map the digital approaches that have been investigated during the past 10 years.MethodsWe searched Medline, Embase, other databases and relevant web sites in July 2018 to identify comparative studies of digital approaches for recruiting and/or retaining participants in clinical or health RCTs. Two reviewers screened references against protocol-specified eligibility criteria. Studies included were coded by one reviewer (with 20 percent checked by a second reviewer) using pre-defined keywords to describe characteristics of the studies, populations and digital approaches evaluated.ResultsWe identified 9,133 potentially relevant references, of which 100, reporting 101 unique studies, met the criteria for inclusion in the map. Among these, 95 percent of studies investigated recruitment but only 11 percent investigated retention. Study areas included health promotion and public health (36 percent), cancer (17 percent), circulatory system disorders (13 percent) and mental health (10 percent). Most study designs were observational (89 percent). The most frequent digital approaches for recruitment were internet sites (53 percent of recruitment studies), social media (42 percent), television or radio (31 percent) and/or email (31 percent). For retention these were email (63 percent of retention studies) or text messaging (38 percent). Time and costs of recruitment were reported in 17 percent and 30 percent of recruitment studies respectively, whilst costs were reported in 19 percent of retention studies.ConclusionsA wide range of digital approaches has been studied, in many combinations. Evidence gaps include lack of experimental studies; studies on retention; and studies for specific populations (e.g. children or older people) and outcomes (e.g. user satisfaction). More robust experimental studies, perhaps conducted as studies-within-a-trial (SWAT), are needed to address knowledge gaps and ensure that estimates of digital tool effectiveness and efficiency are reliable.