The renin-angiotensin system (RAS) is a signalling pathway that acts as a homeostatic regulator of vascular function. Its systemic actions include the regulation of blood pressure, natriuresis, and blood volume control. However, the RAS also plays an important local role, regulating regional blood flow and controlling trophic responses to a range of stimuli. The RAS is composed of a number of different regulatory components and effector peptides that facilitate the dynamic control of vascular function, in both health and disease. Many of these components have opposing functions to accommodate a rapid but coordinated response to specific triggers.