The techniques of tracer dilution in the circulation, and of tracer uptake by and washout from an orgen, may be described using expressions that are general and are not dependent on specific models such as exponentials. The expressions have been applied to the measurement of cardiac output using impulse and constant rate injection techniques. Further expressions have been given for estimating organ blood flow from inflow/outflow concentration-time curves, and from the distribution of deposited tracer. Some problems with respect to the use of deposition techniques as they are ordinarily applied to the estimation of regional blood flow must be considered, particularly when there are capillary beds in series or where there is countercurrent diffusional shunting of diffusible tracers between inflow and outflow. This review deals with these various aspects of tracer theory as they relate to the measurement of blood flow.