This paper reviews empirical studies over the past decades from all over the world in order to assess what researchers have done about audit quality issue and identify gaps in the literature where further research is needed. These studies are categorized into seven groups; 1) Using direct or indirect measure of audit quality 2) Studies based on source of differentiation 3) Studies rely on output, process and input 4) Organizational aspects 5) Behavioral perspectives and auditor quality 6) Different perceptions of audit quality and 7) Other studies. The existing body of knowledge relating to these groups of studies will be summarized in this paper.