Measures of development efficiency are defined for dual component electrophotographic development systems based on the known physics of development. The first definition, useful for all development systems, is the ratio of the toner developed to toner available. It is useful for demonstrating when toner depletion governs the physics of development. For the insulative magnetic brush development system, the second definition of efficiency measures the strength of the electric field as seen by toner particles. Experimentally it is shown that it is closely equal to the dielectric constant of the mix. A theoretical explanation for this result is presented. For the conductive magnetic brush development system, the third definition of efficiency is unity for an infinitely conductive system and decreases as the conductivity decreases. Therefore it is a measure of developer degradation.