High altitude, high speed flight will push vehicles into regions wherein the density of the surrounding medium is so low that vehicle aerodynamics cannot be described on the basis of the continuum equations of fluid motion. Typical flight trajectories and the characteristic flow regions they traverse are illustrated, and the prediction techniques based on molecular flow physics are outlined. Some analytical, experimental, and flight test results which clearly illustrate the importance of low density effects on the flight performance of vehicles -- particularly lift, drag, and moment -- are discussed. The data presented bring out some fundamental physical principles of molecular interactions in the definitions of aerodynamic behavior, and some of the underlying physical mechanisms are discussed. Molecule-to-molecule interaction is only one of the processes which determine flow field characteristics. Molecule-to-surface interaction becomes important at some Knudsen numbers and at high Mach numbers, real gas effects caused by the high temperatures also become important. These are also illustrated. Among the various theoretical and computational approaches which are vigorously pursued today, Monte Carlo methods and the Burnett equations are extensively applied. The bases of these methods are outlined and some of their results are discussed in the paper.