Cognitive behavioural therapy (CBT) is widely acknowledged as a treatment of choice for a range of psychological disorders, including those that affect criminal offenders and mentally disordered offenders. Meta-analyses of multiple studies provide evidence of CBT's efficacy and have led to its recommendation as a first line treatment for anti-social personality disorder (ASPD; NICE, 2009). However, there is comparatively little evidence about the effectiveness of CBT in relation to complex problems faced by offenders in prison and secure psychiatric services. The aim of this study was to examine how clinicians conceptualize and respond to complexity in the practice of CBT treatment with high-risk mentally disordered offenders. Using a qualitative methodology, semi-structured interviews were conducted with six practitioners. The interviews were analysed using thematic analysis. The first main theme was ‘critical challenges and issues’ which included subthemes addressing patient characteristics, the therapeutic context, ethical challenges, and treatment challenges. The second main theme was ‘overcoming obstacles’ which included subthemes on promoting safety, an emphasis on non-specifics, how to create an enabling space and creative practice in the application of CBT. The study emphasizes the importance of creating an enabling safe space by being ‘responsive’ to the patients’ needs and also emphasizes the importance of working to establish a baseline for ‘safety’ in all alliances wherever possible.