This chapter provides detail description of gastroenteritis. Chapter has 6 well illustrated diagrams and pictures. Diarrhoea, rotavirus diarrhoea, assessment of dehydration, algorithm for treating a child, composition of ORS by WHO, types of dehydration based on electrolyte imbalance, isotonic dehydration, hypertonic dehydration, hypotonic dehydration, acidosis, feeding in gastroenteritis and drugs in gastroenteritis are described in detail. 4 tables are included to highlight criteria for severity of dehydration, composition of ORS and types of fluids for infusion. 23 references are included at the end of the chapter to facilitate further reading.