In medicine and public health, the randomised controlled trial (RCT) is generally considered the key generator of 'gold standard' evidence. However, basic and clinical research and trials are often unrepresentative of real-world populations. Recruiting insufficiently diverse cohorts of participants in trials (e.g. in terms of socioeconomic status, racial and ethnic background, or sex and gender) may not only overstate the general effectiveness of a technology; it may also actively increase health inequalities. We highlight some general issues in this domain, before discussing several specific illustrative examples in the context of medical devices. High quality evidence on factors that would improve trial recruitment is extremely limited. There is a clear need for research on candidate strategies for improving recruitment of under-represented groups in RCTs. These could include, for example, offering various forms of financial incentives; non-monetary incentives, such as preferential access to the technologies that are being tested if they are found to be effective; and various types of informational messages and nudges; as well as involvement of community partners and champions in the recruitment process. Ideally, recruitment practices should ultimately be based on evidence generated from RCTs. Studies Within a Trial (SWAT), where randomised experiments are built into the actual recruitment processes in RCTs, are an ideal way to gain this evidence. SWAT studies are seeing an increase in traction, as indicated by funding streams in bodies such as the UK-based NIHR. Making greater funding available for studies of this kind is needed to improve the evidence base on how best to improve diversity in trial recruitment.