NASA’s Advanced Space Transportation Program (ASTP) is investing in technologies to achieve a factor of 10 reduction in the cost of Earth orbital transportation and a factor of 2 reduction in propulsion system mass and travel time for planetary missions within the next 15 years. Since more than 70% of projected launches over the next 10 years will require propulsion systems capable of attaining destinations beyond low-Earth orbit, investment in in-space technologies will benefit a large percentage of future missions. The ASTP technology portfolio includes many advanced propulsion systems. From the next-generation ion propulsion system operating in the 5–10 kW range to fission-powered multikilowatt systems, substantial advances in spacecraft propulsion performance are anticipated. Some of the most promising technologies for achieving these goals use the environment of space itself for energy and propulsion and are generically called “propellantless,” because they do not require onboard fuel to achieve thrust. An overview of state-of-the-art space propulsion technologies, such as solar and plasma sails, electrodynamic and momentum transfer tethers, and aeroassist and aerocapture, are described. Results of recent Earth-based technology demonstrations and space tests for many of these new propulsion technologies are discussed.