Rapid, worldwide implementation of electrical power generation from renewable sources, often located at great distances from urban and industrial consumers, has led to the need for new, low loss, long-distance transmission systems. A result of this trend is that development of DC transmission technologies is progressing rapidly. As a part of this, gas-insulated DC transmission lines (DC GIL) and gas-insulated DC substations (DC GIS) are of particular interest due to their space-saving advantages. Testing of these systems poses unique challenges and forms the primary focus of this Technical Brochure. The technical feasibility and initial application of gas-insulated HVDC GIS was described in CIGRE Technical Brochure 506. Since its publication (2012), a large body of service experience had been collected from the first DC GIS installations, and further projects are expected to be realized soon. All these DC GIS were subjected to thorough dielectric testing, covering the basic phenomena but using different testing procedures and test levels, since no standards for (dielectric) testing of HVDC gas-insulated systems are currently in place. Therefore, CIGRE Joint Working Group JWG D1/B3.57 was created to consider this topic in detail.