This presentation gives an introduction to indoor wireless systems and why propagation models are needed. The speaker discusses: building structures and materials and their influence on the propagating fields; field strength measurements in typical indoor environments; current techniques for modelling propagation indoors; the role of computational electromagnetics in the modelling paradigm and the use of mechanistic models; full-wave field characterisation and power flow visualisation; and the levels of environmental detail that are required for accurate predictions of signal strength.