The experimental observable in a muon experiment is the time dependence of the muon polarization. The polarization function that describes this time evolution depends sensitively on the nature of the material into which we have implanted our muons. The positron asymmetry that we measure in a μSR experiment is directly proportional to this polarization function. We give an introduction to polarization functions, describing the effect of static fields as well as an introduction the effect of dynamics and disorder.