This text highlights some of the pressing issues in the use of modelling techniques for the design of analogue circuits and introduces the reader to two main language standards, VHDL-A and MHDL by using in-depth examples of the use of these languages to model analogue devices. Using models for circuit design gives designers the power to express directly the behaviour of parts of a circuit in addition to using other pre-defined components. There are numerous advantages to this new category of analogue behavioural language. In the short term, by favouring the top-down design and raising the level of description abstraction, this approach provides greater freedom of implementation and a higher degree of technology independence. In the longer term, analogue synthesis and formal optimization are targeted. The final part is devoted to the topic of modelling the thermal and electrothermal aspects of devices.