The main goals of this final thesis are to analyze and determine the market influence of audit firms from the "Big Four" group on the audit services market in Croatia and to compare them with other audit firms according to selected criteria (number of clients, auditors and non-audit fees from entities of public interest). In the introductory chapter, the problem and subject of the research are presented and the direction of the research is indicated. Then the theory of agents and the problem of information asymmetry are theoretically defined, and the conditions that create the demand for audit services are explained. The following are the laws and principles that must be observed when performing the audit profession. In the next chapter, the offer of audit services is described, which is characterized by a high level of concentration, which potentially creates a problem for maintaining the quality of the audit service. The origin of the "Big Four" group through history is explained, and the members and their business characteristics are defined. In the empirical part of the work, the main emphasis was on determining the concentration on the audit services market in Croatia. The research was conducted on the collected data from the transparency report on selected audit firms for the year 2021.