Monolithic scintillator detectors are very promising for high-resolution high-sensitivity PET. These detectors consist of several cubic centimetres of scintillating material coupled on one or more sides to position-sensitive avalanche photo-diode (APD) arrays. The coordinates of the entry point of an incoming gamma photon are estimated from the light distribution on the APD arrays. Modelling the detector can make the optimization of the detector more efficient: understanding of the detector is increases, the parameters that contribute most to the performance can be identified, and less prototypes need to be built. Two methods for modelling PET detectors are presented. The first are Monte Carlo simulations. All the relevant parameters for performing these simulations for monolithic scintillator detectors have been determined and the simulations have been validated using measurements. Second, a model is derived from which the theoretical lower bound on the spatial resolution can be determined.