Optimizing the flow of both liquids and gases is crucial to many areas of anaesthetic and critical care practice. In this article, the physical principles that govern the flow of gases are described and illustrated through examples of their application in clinical practice. The difference between laminar and turbulent flow is explained, along with the factors that determine the nature of fluid flow. The Venturi effect, Bernoulli principle and Coanda effect are also described with reference to their clinical applications and their relevance to medical devices.