Vehicle-to-grid (V2G) systems represent a means by which power capacity in parked vehicles can be used to generate electricity for the grid. This paper describes the first detailed and global analysis of the potential of V2G technologies over the long-term (to 2100) using a comprehensive energy-systems model. In this analysis we explore the potential for V2G systems to supply a number of electricity submarkets and concomitantly accelerate the diffusion of advanced vehicle technologies, including hybrid-electric and fuel cell drivetrains. We also examine the potential impact of V2G on the global energy system, particularly in terms of investment in conventional capacity, and the possible role of V2G-enabled vehicles in increasing the market penetration of renewable electricity generation technologies. Importantly, however, V2G technologies represent a paradigm shift in how the energy and mobility markets are related, and a number of possible barriers to the widespread adoption of this technology are also discussed.