The majority of research using the commonsense model of illness representation has focused upon observable, organic illness; little research has been directed at invisible, functional illness, even though a clear understanding of the cognitive mechanisms in functional illness is warranted. The main purpose of this thesis is to explore the illness cognitions (or illness representation) of different populations of irritable bowel syndrome (IBS) sufferers over time, to examine whether these cognitions can predict simultaneous and subsequent outcome, and to determine if coping mediates the link between illness representation and outcome as the commonsense model suggests. The first two chapters introduce the commonsense model, and discuss why IBS is a worthwhile illness to study in this way. Chapters 3,4,5 and 6 describe each of the four studies conducted in order to examine the commonsense model with IBS sufferers/patients. Sufferers/patients were recruited from three different populations: selfhelp groups, primary and secondary care. Three of the four studies were longitudinal so that the predictive nature of the representation components could be assessed over time. In addition, three of the four studies compared the responses of IBS sufferers to sufferers of other illnesses: inflammatory bowel disease (1131D) (an organic illness) and chronic fatigue syndrome (CFS) patients (a functional non-GI illness). All four studies confirmed that the illness representation of IBS sufferers can predict outcome, and have shown that the representation can predict anxiety significantly 12 months later. All four studies demonstrated the importance of the serious consequence component of the illness representation; reporting that IBS has serious consequences was related significantly to poor outcome. Avoidant coping was found to partially mediate the link between illness representation and outcome but was found to not contribute much to the illness representation. The predictive strength of the illness representation is stronger in IBS sufferers compared to IBD sufferers. While we can hypothesise that the strength of the representation is attributable to the illness' functional status we cannot make any further assessment regarding the role of symptom presentation due to the small number of CFS patients recruited. The final chapter outlines the main findings of the thesis and discusses the salient points. The theoretical and empirical implications of this research are discussed.