A general expression for the time‐independent transmission coefficient of a double barrier is derived. Unity double barrier transmission is found possible for all energy levels above or below the double barrier potential energy levels. The conditions under which unity transmission occurs are obtained and discussed.Application of the general theory to a piecewise constant potential model indicates that double barrier effects in thin film devices may be most noticeable for a majority of current carriers at energy levels near the maximum potential energy level of the double barrier. The small width of the transmission resonances in the examples considered indicates that double barrier effects would probably not be detected, and if they were detected would probably not be interpreted as such, unless investigation of this particular phenomenon was an objective of the research being conducted.