Interventions targeting processes that commonly maintain different psychological disorders have demonstrated promising effectiveness data. However, very few studies have examined brief transdiagnostic groups. Qualitative explorations of the acceptability of both transdiagnostic and brief groups are also very limited. A brief transdiagnostic group, the Take Control Course (TCC), has been developed for clients with common mental health problems in primary care services.This thesis examined the effectiveness and acceptability of the TCC, which is a 6-session transdiagnostic group intervention. The TCC is explicitly theory-driven and targets mechanisms of psychological change specified by Perceptual Control Theory (PCT). Three papers are presented within this thesis that examined: i) the empirical and conceptual background of the TCC; ii) whether the TCC was non-inferior compared to an active control (individual low-intensity Cognitive Behavioural Therapy, CBT); iii) whether the TCC was acceptable to participants using qualitative examination of participants' experience of the TCC and perceptions of what contributed to psychological change (or lack of change).The thesis comprises: i) a narrative review; ii) a non-inferiority RCT; iii) a qualitative interview study using Thematic Analysis. The thesis utilised mixed methods in order to understand the TCC from multiple perspectives. The overall programme of research that the thesis is part of is strongly informed by the Medical Research Council (MRC) framework for developing and evaluating complex interventions. Although the MRC guidelines stress the utility of randomised experimental designs in evaluating an intervention, they also recognise the contribution that qualitative methodologies make to such evaluations.The narrative review outlined how the development of a transdiagnostic intervention, which targets specific transdiagnostic processes, could provide an efficient way of promoting psychological change. It explicitly detailed how the theoretical basis informed intervention components. Within the RCT, intention-to-treat analyses at 6-month follow-up found that the TCC was non-inferior to individual low-intensity CBT on anxiety and depression outcomes, functioning and an idiosyncratic problem measure. 156 clients were randomised. Secondary, per-protocol analyses, found inconclusive evidence of non-inferiority. This was the first randomised trial providing evidence for the non-inferiority of a brief transdiagnostic group compared to established individual therapy. The qualitative study of 12 in-depth interviews indicated that the flexible group format of the TCC was appreciated, as participants felt able to engage at their own pace and adapt components. Greater clarity regarding what was within participants' control reduced distress and enabled effective pursuit of valued goals. This was the first qualitative study of participant experience of a brief transdiagnostic group.Findings indicated that TCC was acceptable and non-inferior to an established CBT intervention. This adds to the evidence base for transdiagnostic interventions.