Therapy with intravenous fluids is an integral part of paediatric critical care. However, no ideal intravenous fluid exists, and it is only very recently that randomised controlled trials have been conducted to evaluate their role in the critically ill child. As our understanding of cell physiology and disease pathophysiology developed, so too has our approach to fluid therapy and a recognition that fluids should be considered like any other drug, maximising efficacy, and minimising disbenefits [1]. This chapter will explore the history of intravenous fluids and fluid physiology and consider the selection, timing, and dose of intravenous fluids in the critically ill child.