The evolution of high-energy density fuels over the past three decades is briefly described. This period can be characterized by exceedingly slow progress and notable lack of success toward the development of practical, economically viable fuel systems. Recently, two novel ultrahighenergy density fuels, one naturally occurring and one synthetic, have emerged; these fuels, which are both composed of compact hydrocarbon molecules, have energy contents or heating values significantly greater than that of currently used standard missile fuel JP-10 (up to 160K Btu/gal (44.7K MJ/m 3 ) vs 141.7K Btu/gal (39.6K MJ/m 3 )). In addition, these fuels also exhibit superior low-temperature, viscometric, flash-point, and other properties that are desired and, indeed, required for practical fuels. Initial research and development of these fuels are described, and their chemistries, properties, and, in the case of the naturally occurring fuel, engine test results are provided.