Using molecules radiolabelled with positron emitters, positron emission tomography (PET) imaging provides information relating to a multitude of physiological and metabolic functions. Whereas primary PET images give a qualitative insight into these functions, when PET images are combined with data of the time course of the PET radiopharmaceutical in the blood, in an appropriate biological model, the extraction of quantified parameters of the observed function becomes possible. This chapter gives an introduction into kinetic modelling used with PET and the related mathematical procedures. Additionally, some basic and often used applications are described and are followed by an overview of the application of kinetic modelling in the case of MR-PET.