About modelling issues, the research on CNTFETs is still at an early stage. Most of the models available in literature are numerical and make use of self-consistency and therefore they cannot be directly implemented in modelling languages for analog and digital circuits, such as SPICE, Verilog or VHDL-AMS. In this chapter we examine the modelling issue, with particular reference to conventional CNTFETs, because they show the best performances in terms of “on-off” ratio currents and subthreshold swing. In particular we review a compact, semi-empirical model of CNTFETs, already proposed by us, in which we introduce several issues to allow an easy implementation in the most common circuit simulators. The CNTFET equivalent circuit is similar to a common MOSFET one, where the quantum capacitances have been computed from the charge in the channel. A procedure, based on a best-fitting between measured and simulated values of output device characteristics, is reviewed in order to extract the optimal values of the CNTFET equivalent circuit elements. Finally we implement our model both in SPICE, using ABM library, and in Verilog-A in order to demonstrate the model flexibility and to compare them.