DIARRHOEA is very common in the calf and can have an impact both economically and in terms of animal welfare. Losses are due to death, treatment costs and time spent on care, as well as subsequent chronic illthrift and poor growth. While it may be convenient to focus on the principal infectious causes of calf diarrhoea, it is important to remember that it is generally the result of interaction between a number of related risk factors. Mixed infections are frequently seen and clinical signs are usually more severe where more than one pathogen is involved. This article discusses the causative agents and risk factors associated with the development of diarrhoea in the calf and describes an approach to the investigation of clinical disease.