This thesis is submitted in partial fulfilment of the requirement for the degree of Doctorate of Clinical Psychology (DClinPsy). It is a portfolio thesis and as such consists of three separate papers. The first paper is a systematic review that aimed to examine whether Positive Behavioural Support (PBS) is effective for adult service users. The second paper is an empirical research study in which the validity of the English version of the Forensic Quality of Life Questionnaire - Short Version was explored using a cross-sectional qualitative research design. The third and final paper is a critical evaluation of both the systematic review and the empirical research study. The systematic review and empirical research study have been prepared in accordance with the author guidelines of a targeted journal for publication.