Intra-arterial cannulae in the radial artery are used for invasive arterial blood pressure (IABP) measurement and for collection of blood for analysis. The radial artery is the preferred site for insertion because of low complication rates. Arterial lines are the gold standard for accurate blood pressure (BP) measurement. They may be used in intensive care units (ICU) and high dependency units and in anaesthetized patients undergoing surgical procedures. An understanding of basic principles enables arterial lines to be used safely in these settings.