In this thesis, implantation of radioactive isotopes and photoluminescence studies were used to identify defects in ZnO. Implantations of stable and radioactive isotopes of gallium and zinc were performed. From photoluminescence studies of these samples, a feature at 3.3600 eV was shown to be due to recombination at a Ga donor site, while a feature at 3.3225 eV was shown to follow the concentration of the daughter product Ge. A broad red band was found to grow in samples implanted with 72zn. This is believed to be a damage related band, created by the high recoil energy of the 72ge daughter product. Radioactive isotopes of Cu and Ni, with Cu as a decay product, were implanted into single-crystal ZnO in order to investigate the relationship between Cu and the structured green band, a common feature seen in photoluminescence spectra of ZnO. As the intensity of the green band was found to increase with the half-lives of the Cu and Ni isotopes, no correlation could be obtained between the Cu concentration in the samples and the green band. An explanation for these results was found to lie with the high recoil energy of the daughter products of the implanted isotopes, and the green band was attributed to the creation of Zn-vacancies. The temperature dependence of new signals observed in the near IR was examined. These signals were also studied using photoluminescence excitation measurements. Attempts to decipher the excited states of the new emission features were made through manual subtraction of the contributions of the Fe3+ phonon replicas, and also examination of the PL intensities at various excitation energies. A possible model for the excitation mechanism of the previously reported Fe3+ emission is presented.