The execution of an HVDC scheme will require a considerable number of system studies to be undertaken throughout the timeline of the project development, by the owner/operator of the scheme and the suppliers of the HVDC converter stations and the transmission system. This chapter will focus on the studies which will need to be undertaken for the planning of the HVDC scheme. Such studies are normally performed by the Transmission System Operator (TSO) or Network Operator (NO), although some schemes may be developed by private owners who may undertake or commission the necessary studies. The introduction Sect. 34.1 reviews the factors which will need to be considered when developing the needs case for the scheme and the aspects which will need to be studied to define the requirements of the HVDC scheme. Section 34.2 provides an overview of the planning studies which will need to be undertaken by the TSO/NO to determine the parameters of the proposed HVDC scheme and its integration into the AC transmission networks. More details of the planning studies for HVDC schemes are provided in Sect. 34.3. This presents the timelines for the necessary studies, considering long term (3–10 year) and medium term (0–3 year) scenarios, with the study objectives, input data and study types being clarified. The types of power system studies are discussed together with the impacts of the HVDC scheme on the AC networks. Section 34.4 discusses the studies which will be required for the preparation of a technical specification for the HVDC scheme, which will be issued to the market for competitive tendering. These studies will clarify the power rating of the scheme and its topology. The studies will require detailed information on the representation of the AC networks at the time of initial operation and future scenarios, including information on short circuit levels and network harmonic impedances. It may be necessary for the TSO/NO to undertake transient stability and electromagnetic transient studies to ensure that the proposed HVDC scheme has no adverse impacts on system stability and demonstrate its technical feasibility. Finally, Sect. 34.5 provides a detailed discussion of the digital models which will be needed to undertake the required planning studies discussed in Sect. 34.4. A key aspect is the availability of models of the HVDC controller, as generic models available in commercial study tools may need to be used with some caution. For each type of study, the capability of the model, its operating aspects, and the use of the model are described.