This chapter explores the crucial role of simulation and modelling of electronic and photonic components for terahertz (THz) systems. THz-related challenges already begin with setting up the signal generation and sampling parameters and continue with the realistic modelling of the electronic and photonic building blocks. Hereby, photonic components require not only the modelling of the optical signal propagation but also the modelling of the electronic interface in the THz regime. Furthermore, when advancing to the simulation of systems like fully integrated electronic transmit and receive frontends or photonically assisted analogue-to-digital converters (ADCs), it is up to the designer to find a suitable level of abstraction. Size, complexity, and available computational power versus accuracy must be taken into consideration and prioritized against each other.