Background: There is a great need for effective treatments for mental health problems. Randomised controlled trials are the gold standard for evaluating treatments, however recruitment into trials is challenging, highlighting a clear need for evidence-based recruitment strategies. This thesis aimed to systematically develop a recruitment intervention and evaluate its effectiveness for improving the recruitment of participants into mental health trials.Methods: A mixed-methods approach, adopting the Medical Research Council's complex interventions framework: 1) a systematic review to identify the evidence base and describe the factors affecting recruitment into depression trials; 2) a qualitative study to understand patients' decision-making process in declining to enrol in a depression trial; 3) development of a recruitment intervention, using Participatory Design methods; and 4) evaluation of the recruitment intervention, using a randomised controlled trial, embedded in an ongoing mental health trial (the EQUIP trial). The primary outcome was the proportion of participants enrolled in EQUIP. Results: From the systematic review, a conceptual framework of factors influencing the decision to participate was developed, which highlighted that the decision to enrol involves a judgement between risk and reward. Findings suggested that patient and public involvement in research (PPIR) might be advertised to potential participants to reduce such perceived risk. The qualitative study found positive views of trials. Interviewees' decision making resembled a four-stage process; in each stage they either decided to decline or progressed to the next stage. In Stage 1, those with an established position of declining trials opted out - they are termed 'prior decliners'. In Stage 2, those who opted out after judging themselves ineligible are termed 'self-excluders'. In Stage 3, those who decided they did not need the trial therapy and opted out are termed 'treatment decliners'. In Stage 4, those who opted out after judging that disadvantages outweighed advantages are termed 'trial decliners'. While 'prior decliners' are unlikely to respond to trial recruitment initiatives, the factors leading others to decline are amenable to amelioration as they do not arise from a rejection of trials. We recruited a host mental health trial (EQUIP), and worked with key stakeholders, including mental health service users and carers, to develop an intervention using a leaflet to advertise the nature and function of the PPIR in EQUIP to potential trial participants. 34 community mental health teams were randomised and 8182 patients invited. For the primary outcome, 4% of patients in the PPIR group were enrolled versus 5.3% of the control group. The intervention was not effective for improving recruitment rates (adjusted OR= 0.75, 95% CI= 0.53 to 1.07, p=0.113). Conclusions: This thesis reports the largest ever trial to evaluate the impact of a recruitment intervention. It also reports the largest trial of a PPIR intervention and makes a contribution to the evidence base on trial recruitment as well as to that assessing the impact of PPIR. Two further embedded trials are underway to evaluate the effectiveness of different versions of the recruitment intervention in different trial contexts and patient populations. This will also allow the results to be pooled to generate a more precise estimate of effect; to evaluate the impact of the intervention on trial retention; and to explore patient experiences of receiving the intervention.