An examination of the binary boundary-layer problem in connection with providing better estimates of surface-cooling methods for use with hypersonic vehicles, such as ICBM nose cones. A binary boundary layer is one in which some foreign substance has been injected to alter the properties of the flow, notably its heat-transfer characteristics. The mechanism of laminar binary boundary-layer flow is discussed in mathematical terms, and five different analyses involving a variety of injected substances are reviewed. Generalized expressions are then developed for predicting heat-transfer and skin-friction performance in the presence of mass-transfer cooling for laminar flow over a flat plate. The results indicate that different foreign materials (e.g., hydrogen, carbon dioxide, and iodine vapor) injected into the boundary-layer stream reduce heat-transfer and skin-friction coefficients by an amount that depends on the molecular weight of the injected material. In conclusion, mass-transfer cooling in a turbulent boundary layer and sublimation cooling are considered.