During the 1980s and 1990s the U.S. sponsored a number of space nuclear power and propulsion programs and studies covering many different mission plans. These space nuclear activities spanned the range from the 300-watt radioisotope thermoelectric generators currently in use on the Galileo mission to Jupiter and the Ulysses mission to explore the polar regions of the Sun to multi-megawatt space nuclear reactor concepts that could be used to provide power for neutral particle beams, free electron lasers, electromagnetic launchers, and orbital transfer vehicles. Studies were undertaken on both nuclear electric propulsion (NEP) and nuclear thermal propulsion (NTP) largely in support the proposed Space Exploration Initiative (SEI). Upgrades to radioisotope power sources involving dynamic (turbine-alternator and linear oscillator) and improved static (thermophotovoltaic and alkali-metal thermal-to-electric conversion) conversion systems were investigated. A major undertaking in the 1980s and early 1990s was the SP-100 space nuclear reactor power system which was designed to span the power range from 10 kWe to 1000 kWe to encompass most high-power missions then being studied. In addition, various efforts to explore the uses of thermionic reactors were undertaken, including an initiative with Russia to test the ENISY (TOPAZ II) thermionic reactor. This paper summarizes these activities and highlights future trends in an era of smaller spacecraft that will be built and launched in shorter periods of time under more tightly constrained budgets.