Background People with obsessive-compulsive disorder (OCD) and their relatives suffer reduced quality of life and impairments to wellbeing. The importance of personal understandings of illness (illness perceptions) in determining condition management behaviours and illness outcomes is recognised in physical health, though less widely in mental health. No study has conducted a detailed investigation of the role of illness perceptions in OCD. This thesis reports research which describes illness perceptions in people with OCD (POCD) and their relatives, develops a new measure, and tests correlations between illness perceptions and important outcomes. Methods A systematic review to determine factors associated with variation in burden and distress in relatives was carried out. Three empirical studies were conducted. Two parallel qualitative studies involving POCD (N = 16) and their relatives (N = 14) were undertaken to develop a detailed understanding of how OCD is understood by each group. Transcripts from semi-structured interviews were analysed using thematic analysis. Based on interviews with POCD, items representing key additional dimensions of illness understanding were generated to supplement a generic measure of illness perceptions. A new Illness Perceptions Questionnaire for OCD (IPQ-O) was developed. Patient-public involvement (PPI) ensured its comprehensiveness, acceptability and clarity. The IPQ-O was completed, together with other questionnaires, by 348 adult POCD in a cross-sectional study. The psychometric properties (factor structure, reliability, internal consistency) of the IPQ-O were described. Construct validity was established via correlations with other measures. Results Fifteen studies using quantitative (n=12), qualitative (n=2) and mixed methods (n=1) were included in the systematic review. The disparate factors investigated and cross-sectional study designs made it difficult to identify factors leading to burden/distress, though qualitative evidence indicated that unmet support needs (e.g. knowledge gaps) were a source of burden in relatives. Findings from the qualitative studies showed that some aspects of understanding mapped onto the key dimensions of illness perception identified in physical health, although other aspects of perception markedly differed. In particular, POCD and their relatives see OCD as a permanent âpartâ of the person. Perceptions of OCD symptoms as on a spectrum of âusualâ behaviour suggested that OCD was seen by some as a trait-like condition. Factor analysis of the IPQ-O identified a seven-factor structure of the main scale. Illness perceptions were associated with emotional and behavioural outcomes in OCD. In particular, people who had not taken medication for OCD scored significantly higher on the spectrum scale compared with those who had received it. Conclusions To improve outcomes in OCD, it may be important to modify illness perceptions associated with poor coping and adjustment. Longitudinal study is now needed to test whether illness perceptions drive outcomes as predicted.