Heart failure is a chronic medical condition increasing in prevalence. It can be treated effectively, yet outcomes for patients remain poor. Research suggests that this is because many patients do not adhere to recommendations regarding self-management. It has been proposed that this may result from a poor conceptual grasp of the illness, understood according to Leventhal’s self-regulatory model as a lack of coherence. A mixed methodology study was conducted to explore coherence in heart failure. Thirty-eight outpatients with heart failure completed a measure of illness perceptions, mood and belief in the effectiveness of health behaviours. Semi-structured interviews were conducted with nine, selected according to whether their perceived illness coherence was high or low. Pervasive gaps in knowledge were evident regarding appropriate self-management. Patients who reported a highly coherent model understood their heart failure as a chronic condition and were better able to accept the limitations it imposed. Patients who reported a low coherence model were confused about the diagnosis, and their accounts were dominated by concerns about symptoms and treatment side-effects. They were more likely to take a passive role in their healthcare, with implications for self-management. Social barriers to the development of a coherent model were identified. The results suggest a role for clinical psychology in supporting the development of a coherent illness model through interventions aimed at reducing anxiety and facilitating communication.