While people with intellectual disabilities (IDs) are at increased risk of developing mental health problems, they are disadvantaged when accessing mental health services. The aims of this thesis are threefold: 1) to evaluate the efficacy of psychological therapies for people with IDs who experience mental health problems, 2) to improve the suitability of adults with IDs for cognitive behavioural therapy (CBT), and 3) to explore the acceptability of computer programmes in therapy as a reasonable adjustment to improve therapy accessibility. A systematic review and meta-analysis of the current literature were conducted and identified CBT and individual therapies as the most efficacious treatment for anger and depression. Although adults with IDs and concurrent mental health problems appear to benefit from psychological therapies, clinical trials need to make use of improved reporting standards and larger samples. Next, in two subsequent single-blind mixed experimental designs the efficacy of computerised training programmes in improving CBT skills in people with mild to moderate IDs is evaluated. Training programmes focused on linking situations to feelings and discriminating between thoughts, feelings and behaviours. When compared to an attention-control condition, training improved cognitive mediation skills, as assessed by the ability to link situations and mediating beliefs to feelings, and improved the ability to differentiate between thoughts, feelings and behaviours. In a concluding qualitative study, the perspectives of service users with IDs and clinicians on using computers in therapy were explored. Both service users and clinicians were positive about the potential functions and benefits of using computers in therapy, but also drew attention to potential challenges and barriers. Together, these studies show that computers can be used to improve the suitability of people with IDs for CBT and meanwhile encourage further exploration into the possibilities that these technologies can open up for improving the accessibility of psychological therapies.