A hybrid density functional study based on a periodic approach with localized atomic orbital basis functions has been performed in order to compute the optical and thermodynamic transition levels between different charge states of defect impurities in bulk ZnO. The theoretical approach presented allows the accurate computation of transition levels starting from single particle Kohn-Sham eigenvalues. The results are compared to previous theoretical findings and with available experimental data for a variety of defects ranging from oxygen vacancies, zinc interstitials, and hydrogen and nitrogen impurities. We find that H and Zn impurities give rise to shallow levels; the oxygen vacancy is stable only in the neutral V(O) and doubly charged V(O) (2+) variants, while N-dopants act as deep acceptor levels.