People react to illness in a manner that reflects a complex and evolving interaction of several factors. Most illnesses, except the very trivial, require a period of adjustment and a re-appraisal of lifestyle, ability to work and engagement in leisure activities. Potentially fatal and rapidly progressive disease might be expected to result in more severe psychological reactions, but the evidence indicates that there is surprisingly little correlation between psychiatric morbidity and disease severity when this has been measured in a standardized manner. Metabolic changes accentuate the emotional response to illness. Effective communication is an essential element of good clinical practice. Conversely, poor communication is a source of frustration for patients, leading to uncertainty, resentment, anxiety and depression. Depression resulting from the emotional impact of an illness follows an appraisal of the implications that the illness has on relationships, lifestyle, work prospects, long-term disability and mortality.