Following critical illness patients suffer from significant physical and non-physical sequelae. The aim of the programme of research presented in this thesis was to explore the effectiveness of a programme of exercise in patients discharged from hospital after critical illness. In the absence of an existing intervention, a programme of exercise was developed through synthesis of evidence from a range of sources underpinned by key elements of exercise prescription and provision. The programme consisted of an individually tailored, 6- week exercise programme, supervised by a trained physiotherapist and included a patient exercise manual. A randomised controlled trial (RCT) to investigate the effectiveness of the programme of exercise compared to standard care was rigorously developed and conducted. Sixty participants across Northern Ireland were randomised. The exercise programme did not show significant improvement in the primary outcome measure of patient-reported physical function. However, there were significant improvements demonstrated in a number of secondary outcomes of exercise capacity, health-related quality of life, self-efficacy and behaviour change. Qualitative semi-structured interviews of participants were also conducted and provided an in-depth insight of their perceptions of the intervention, including their intense satisfaction and endorsement of the intervention, perceived benefits, and important facilitators and barriers to impact. The programme of research presented in this thesis contributes knowledge to the limited evidence-base for rehabilitation following critical illness and highlights the real gap in suppOli for these patients. Despite the failure of the programme of exercise to show significant improvement in the pre-defined primary outcome measure, the benefits achieved in important secondary patient-centred outcomes, and qualitative exploration are encouraging and warrant further research. This thesis provides important implications for future research and clinical practice, relating to the target population, recruitment, intervention components, and outcome measures, to improve outcomes for this patient population.