The effects of non-unity Lewis numbers of fuel and oxidizer on the structure of diffusion flames are studied for a variety of flow configurations. By using a generalized formation for the three-dimensional flame-sheet combustion, it is shown that the flame-sheet temperature is increased/decreased with decreasing/increasing Lewis number of either reactant, and that blowing reduces/aggravates these non-unity Lewis number effects if it is parallel/opposing the direction of diffusion. Ignition and extinction analyses are also conducted, yielding explicit criteria governing the occurrence of these events. The falsification of empirically-determined activation energy with the unity Lewis number assumption is emphasized.