Loneliness and social isolation are major determinants of mental wellbeing, especially among older adults. The effectiveness of interventions to address loneliness and social isolation among older adults has been questioned due to the lack of transparency in identifying and recruiting populations at risk. This paper aims to systematically review methods used to identify and recruit older people at risk of loneliness and social isolation into research studies that seek to address loneliness and social isolation.In total, 751 studies were identified from a structured search of eleven electronic databases combined with hand searching of reference bibliography from identified studies for grey literature. Studies conducted between January 1995 and December 2017 were eligible provided they recruited community living individuals aged 50 and above at risk of social isolation or loneliness into an intervention study.A total of 22 studies were deemed eligible for inclusion. Findings from these studies showed that the most common strategy for inviting people to participate in intervention studies were public-facing methods including mass media and local newspaper advertisements. The majority of participants identified this way were self-referred, and in many cases self-identified as lonely. In most cases, there was no standardised tool for defining loneliness or social isolation. However, studies that recruited via referral by recognised agencies reported higher rates of eligibility and enrolment. Referrals from primary care were only used in a few studies. Studies that included agency referral either alone or in combination with multiple forms of recruitment showed more promising recruitment rates than those that relied on only public facing methods. Further research is needed to establish the cost-effectiveness of multiple forms of referral.Findings from this study demonstrate the need for transparency in writing up the methods used to approach, assess and enrol older adults at risk of becoming socially isolated. None of the intervention studies included in this review justified their recruitment strategies. The ability of researchers to share best practice relies greatly on the transparency of research.