This chapter describes the general features of the fish cardiovascular system, highlighting the latest ideas, advances, and research in the field, and details some of the integrated responses of this system to changes in ambient and internal conditions. The arterial system must balance tissue blood supply with tissue metabolic demands. The sympathetic component is of paramount importance for the regulation of blood pressure, cardiac contractility, and distribution of blood flow among vascular beds. For freshwater fish that experience near-freezing temperatures in winter, cardiac physiology must be adjusted to accommodate the cold temperature–driven effects on cardiac excitability and contractility, blood viscosity, and associated vascular resistance. The fish cardiovascular system is also increasingly being used as a model for human cardiac development and function as well as a tool to investigate questions relevant to human health and various oxygen deprivation–related diseases.