Abstract: In patients who cannot, must not or do not want to drink, hydration is a real artificial nutrition intervention. Different clinical situations require a specific approach for fluid administration to reduce the risk of tissue hypoperfusion or oedema. The variations of plasma volume depend not only on the amount of the fluid infused, but also on its distribution, which is determined by the membranes' properties. Fluids for intravenous therapy are not vehicles but real drugs and the knowledge of their properties should guide the choice: the available data on mortality and major complications do not demonstrate the best efficacy between crystalloids or colloids and between the different types of colloids: saline solution at 0.9% is commonly defined as “physiological”, but has significant differences with plasma; balanced electrolytic solutions seem to give better clinical outcomes compared to saline, which is ultimately overprescribed. 5% glucose solution is equivalent to water because glucose is rapidly metabolised, releasing it. It is necessary to strengthen educational interventions to improve the management of fluid therapy.