The rationale for using computer modelling as a technique for investigating systems in electrical impedance tomography is explained. The methodology of modelling is described along with the hazards concomitant with the modelling process. New software tools are currently being used to perform ac analysis over a range of excitation frequencies and the initial results from these analyses are compared with previous results obtained with static models. A new descriptor for the visibility of an object is proposed and the behaviour of this measure with anomaly radius, conductivity contrast and frequency is described