question: Abstract Diarrhoeal diseases are among the most important health problems in the world today. While they are generally mild, nuisance illnesses in the developed world, where clean water and sanitation are adequate, these diseases are the most common cause of death in young children in the developing world, where clean water is not available and sanitation is inadequate. In these areas, up to 25% of children may never reach their fifth birthday. In addition, although the impact on growth and nutritional status is difficult to quantify, the cumulative toll on overall child health is enormous. A wide variety of infectious agents are implicated in the aetiology of acute diarrhoeal illness, many of which were unknown 20 years ago. The epidemiology, pathogenic mechanisms and clinical presentation of the common bacterial and viral pathogens, as they are currently understood, will be discussed in this chapter.