SUMMARYMain modelling approaches used for investigating the Photonic bandgap (PBG) devices are reviewed. In particular, the model based on Leaky Mode Propagation (LMP) method is described.A complete analysis of the propagation characteristics, including the determination of modal propagation constants, electromagnetic field harmonics and total field distribution, transmission and reflection coefficients, total forward and backward power flow in the structure, guided and radiated power, and total losses, can be carried out by a computer program based on the LMP approach.The numerical results have been validated by comparisons with those obtained by using other more complex and expensive models.The new model shows some significant advantages in terms of very low computational time, absence of any a priori theoretical assumptions and approximations, capability of simulating the actual physical behaviour of the device and fast determination of the bandgap position.