Abnormalities of fluids and electrolytes are very common in childhood illnesses. Pediatricians and registrars need to know how to clinically assess a child’s fluid status, the types of fluid and electrolyte changes commonly seen in different illnesses, and how to correct them. In this session we will cover fluid and electrolyte physiology in children, including the water compartments in the body, the role of anti-diuretic hormone (ADH), and the renin-angiotensin system. We will discuss the composition of the different types of intravenous fluids used in paediatrics, how to calculate fluid replacement in children with dehydration, how to make up a 5% glucose solution from plain Hartmann solution or 0.9% NaCl, and the management of hyponatraemia, hypernatraemia, and hypokalaemia.