We have developed a theoretical formalism to determine concentrations of native defects and impurities in semiconductors, and applied it to the problem of p-type doping in ZnSe. Limitations in the achievable hole concentrations are not due to native defect compensation. Two mechanisms are responsible: one is the competition between various substitutional and interstitial configurations, the other is the solubility limit imposed by formation of other phases. A comprehensive examination of Li, Na and N acceptors in ZnSe is presented. © 1992.