There has been an ongoing search for more efficient power plant designs over the last few decades. For fossil-fueled power plants, this has resulted in the development of supercritical water Rankine steam cycles in the 1960s and most recently ultra-supercritical water power cycle systems. In addition, the use of supercritical fluids has been proposed for power cycles as part of the Generation IV (Gen-IV) advanced nuclear reactor designs, since these systems can also provide for higher thermal efficiency and reduced overall costs. For either of these power plant designs, both supercritical water and supercritical carbon dioxide have been considered as working fluids for either Rankine or Brayton cycle designs for a wide range of Gen-IV reactor designs, e.g., supercritical water reactor, high-temperature gas-cooled reactor, and liquid-metal-cooled reactor. In all of these designs, it has become quite apparent that research and development (R&D) investment in innovations in supercritical fluid thermal hydraulics and related materials issues is required to advance the state of the art in more efficient, cheaper, and safer nuclear power system technologies. One can view supercritical fluid transport phenomena as a base technology R&D need that requires more fundamental understanding in a number of areas. The Wisconsin Institute of Nuclear Systems at the University of Wisconsin-Madison has been investigating a range of key phenomena in supercritical fluids involving flow stability, critical flow phenomena, heat transfer enhancement and degradation, as well as materials corrosion issues. This paper summarizes our efforts in thermal hydraulics in order to provide a context for base technology R&D in supercritical fluids to advance Gen-IV systems.