Diarrhoea is a common symptom in all communities and in both general and hospital practice. Diarrhoeal diseases, particularly of infectious aetiology, are a huge health problem world-wide, causing much morbidity and mortality, especially amongst children. Although the symptom is almost universally understood, the definition of diarrhoea is difficult because of the wide variation in the bowel habits of normal individuals. Diarrhoea is, perhaps, best described as a change in bowel habit from normal with an increase in stool volume and/or fluidity with or without an increase in stool frequency. The disorders causing diarrhoea are many and various, but most episodes of diarrhoea are mild and self-limiting. However, all cases of chronic diarrhoea and all severe cases of acute diarrhoea require investigation after the initial clinical assessment. Investigation must be logical and structured and can be divided into three stages: the initial work-up (sigmoidoscopy, stool examination, screening blood tests), anatomical and functional assessment of the gastrointestinal tract, and further investigation of the difficult case (osmotic or secretory diarrhoea?, hormone levels, tests for laxative abuse, perfusion studies, laparotomy, etc.) Most cases are diagnosed after clinical assessment or the initial work-up. In the remainder, there are usually clues to the diagnosis or to the area of the gastrointestinal tract which needs to be investigated. Only a small number of cases require extensive investigation including the third stage of work-up. Analysis of the symptom of diarrhoea requires all the attributes of the good physician: wide clinical experience, careful history and examination, diagnostic and therapeutic acumen, a sound understanding of normal and abnormal physiology, skill and experience in selecting the appropriate investigations and interpreting their results, meticulous attention to detail and finally, a caring and sympathetic attitude to the patient.