This is a discussion of methods for describing, mathematically, flows between compartments in a multi-compartment system. We will give the con ventional theory, based on the solution of a system of linear differential equations; we will also give a theory based on probability, viewing the system as a collection of 'states' with a particle moving from statto state with certain probabilities, remaining in each state a random time with an exponential distribution. Finally, we will take still another approach, again based on probability theory, in which we consider the sojourn time of a particle, that is, the time it spends after leaving a given compartment before returning to that compartment.