In the year 2022 there are over 150 HVDC systems in operation around the world. Approximately more than 70 of these systems are 20 years old or older. Major component of the HVDC systems, e.g. the converter transformers, thyristor valves, reactors and harmonic filters typically have a maintainable lifetime of 35–40 years. As a result, the major components do not need replacement during the lifetime of the HVDC transmission system. However, some subsystems (e.g. control and protection) have lifetimes anywhere between 7 years and 20 years. These subsystems will have to be replaced during the lifetime of the HVDC transmission system. Since the lifetimes of these subsystems varies, it is often challenging to decide which subsystems to replace during a refurbishing project. This chapter describes a process that can be followed to assist in making the decision to replace these subsystems, preparing the technical specification and the bidding process. The issues related to environment, maintenance and commissioning are not included in this chapter. However, the availability of a strategic spare parts supply will influence the feasibility of having a maintainable system, which may extend the life of the system before major refurbishments will be needed.