The Pennsylvania State University has actively pursued hybrid rocket fuel regression rate and combustion research for 15 years. Initial work focused on developing and testing a high-pressure slab-burning hybrid motor with X-ray diagnostics for characterizing the local, real-time regression rates of hydroxyl terminated polybutadiene (HTPB) with gaseous oxygen. Fuel decomposition and pyrolysis investigations were also carried out. Follow-on work is continuing to investigate high regression rate hybrid fuels with various metal additives in center-perforated hybrid motors using both HTPB and paraffin binders. The addition of aluminum powders to paraffin-based solid-fuel formulations was shown to increase the regression rates by a factor of 4 compared to neat HTPB. This regression rate increase corresponds to a mass-burning rate increase ∼7 times that of HTPB when the increase in fuel density is considered. This article reviews some of the more significant results from previous investigations and presents recent data from on-going test programs.