The paper presents the current status of the theory of bound-electron g factor in highly charged ions. The calculations of the relativistic, QED, nuclear recoil, nuclear structure, and interelectronic-interaction corrections to the g factor are reviewed. Special attention is paid to tests of QED effects at strong coupling regime and determinations of the fundamental constants. a) Invited paper published as part of the Proceedings of the Fundamental Constants Meeting, Eltville, Germany, February, 1-6, 2015.