A balance between the volume of fluid taken in by the human body, and the volume of fluid excreted, is essential for life. Body fluid balance, which is maintained via various homeostatic mechanisms, can be disrupted by injury or disease. Prompt action is usually required to replenish fluid volumes and restore homeostasis, which is achieved via intravenous (IV) fluid therapy. Nurses will often encounter patients with a disrupted fluid balance, particularly in critical care. They will be involved in assessing patients’ fluid status and administering and monitoring therapy. Therefore, nurses have an important role in ensuring the safety and effectiveness of IV fluid therapy. This article provides an overview of the principles and aims of IV fluid therapy. It also explains the physiology of body fluid and mechanisms of fluid balance regulation, outlines the principles of patient assessment and indications for IV fluid therapy, and details its potential risks and complications.