Factory testing of HVDC Control and Protection (C&P) Systems is critical to the success of an HVDC project. A thorough test program can validate the design and quality of components as well as their installation. In general, testing in a controlled factory setting allows for more cost effective and flexible testing than a site environment. A thorough factory test program of both components and critical sub-systems can substantially mitigate risks associated with design and manufacturing quality and help ensure successful long-term operation on site. It also will reduce the time required for the system tests since the effort associated with “debugging” the control and protection systems on the site should be substantially reduced. The goal of factory testing is to ensure the equipment or sub-systems are manufactured correctly, and to validate performance under conditions that cannot be practically replicated on site. There are cases for both C&P systems and primary equipment where an off-site test environment can generate conditions that are impractical to generate in a site commissioning environment e.g., comprehensive C&P software performance validation, redundancy checks and C&P response to system contingencies. This chapter provides an overview of the factory testing process for a HVDC project, and then goes into the typical approach and overall proves required to be implemented for factory testing of primary equipment and the HVDC C&P systems and for the dynamic performance tests to be applied to the overall HVDC system. This chapter also covers the factory testing of cooling systems and the testing of offshore HVDC systems.