A few multi-terminal direct current (MTDC) systems are in operation around the world today. However, MTDC grids overlaying their AC counterpart might a reality in a near future. The main drivers for constructing such direct current grids are the large-scale integration of remote renewable energy resources into the existing alternative current (AC) grids, and the promotion and development of international energy markets through the so-called supergrids. This paper presents the most critical challenges and prospects for such emerging MTDC grids, along with a foreseeable technology development roadmap, with a particular focus on crucial control and operational issues that are associated with MTDC systems and grids.