Understanding the physiology of fluid distribution in the human body is fundamental to good clinical practice in anaesthesia and intensive care. Intravenous fluid therapies have a range of clinical and metabolic consequences and they should be context and patient specific. Inadequate or excessive fluid treatment is harmful to patients. There are numerous trials, both historical and current, investigating best practice in fluid therapy. New paradigms and guidelines are being published, and it is important for clinicians to keep up to date with current practice. There is a continued drive to improve the safety of donor blood and prevent transfusion errors. Knowledge of how blood products are collected, separated and stored is essential to prevent harm to patients through transfusions. The development of blood substitutes is progressing, with NHS trials due to start soon.