Patients who come to clinical consultation for chronic diarrhoea (i.e., diarrhoea lasting for more than four weeks) may suffer from a wide range of clinical conditions. The possible diagnoses range from a misunderstanding of what can be considered normal and what pathological in terms of daily bowel movements, to a severe malabsorption syndrome. Since the list of possible causes of chronic diarrhoea can be puzzling, the physician's approach needs to be systematic and structured in order to allow the correct diagnosis and treatment. This article proposes an algorithm for the diagnosis of chronic diarrhoea and discusses in detail the key clinical aspects of celiac disease, which is considered a paradigmatic disease as regards chronic malabsorptive diarrhoea.