This chapter introduces fundamentals relating to the interactive modelling concept. Interactive modelling is a facility available in the Simulink® software. Interactive modelling is suitable for use in academic research and virtual power electronics laboratories in universities, as well as in a power electronic converter design and manufacturing environment. Imagine a power electronics industry manufacturing different types of DC to DC converters. With component values designed by the industry for each converter specification using the well-known modelling equations for the buck converter, the industry can easily and quickly test the performance of each of the buck converters using an interactive model. The entire model is developed using the electrical block set in the Simscape foundation library and Semiconductor block set in the power systems library. With Simscape and Simulink block set components, S to PS converters and PS to S converters have to be connected.