In this chapter we meet a general method for the treatment of the kinetics of a whole system in which the behaviour of substances can by described mathematically in terms of compartments. We will apply the principles to the behaviour of drugs and other externally introduced agents in the form of pharmacokinetics. We will also examine how we can use the same principles in the form of tracer kinetics to measure the delivery of a contrast agent to tissue in medical imaging applications. We will introduce common forms of compartmental models, writing down and to solving differential equations for simple systems. We will see how models are built around ADME principles in pharmacokinetics problems and do similarly for simple tracer kinetic problems.