Temperature-dependent Hall-effect measurements have been used to determine donor and acceptor concentrations and energies in n-type and p-type ZnO. Commonly observed donor energies in n-type material grown from the vapor phase (VP) are 35 and 65 meV, with the 35-meV donor identified as H, and the 65-meV donor, probably Al. Total donor concentrations in VP-grown material are as low as mid-10(16) cm(-3). The only acceptors unambiguously identified so far in this material are the Zn vacancy, at the low-10(15)-cm 3 level, and substitutional No, at a higher level but evidently passivated with H. Acceptors that have been successfully used to create p-type ZnO by doping include N, P, and As, and resistivities as low as 0.1 Omega-cm have been obtained. The acceptor energy for N is about 90 meV for [N] similar to 10(19) cm(-3), and can be estimated to be about 130 - 150 meV at low concentrations of N.