This chapter provides a description of the modelling tools and their modelling capability, primarily as required for studies of the HVDC control and protection (C&P) systems during the lifecycle of the HVDC system. Not all studies referred in this section may be necessary to perform, since they may relate to special project issues e.g. different or unusual HVDC system configurations, different AC system characteristics at the point of connection of the HVDC converters or different line or cable topologies that may need to be considered. The study tools described include both frequency and time domain study tools. Frequency domain studies include simple load flow study tool up to the quasi-stationary radio frequency domain. Time domain study tools might be tools used for transient stability studies up to the study of lightning surges. The use of real time simulator for study of the performance of the HVDC systems in response to disturbances that might require the use of a digital system replica to assess the likely response to the disturbances, is also covered in this chapter.