Among the most important mechanisms of corporate governance, we can single out on one side the audit committee and on the other side the external audit and the process of external audit that the auditor goes through during the audit of financial statements. The focus of this research is the analysis of issues from the Corporate Governance Code related to the work of the audit committee and based on which was calculated Auditing Board effectiveness index for the period from 2015 to 2018 on a sample of 124 companies in 2018 and 137 companies in previous years and annual audit reports from which items related to the auditor's opinion, going concern, key audit issues and the audit company that performed the audit on the same sample for 2018 were specifically observed. Thus, the aim of this study is to explore the role and importance of the audit committee in the external audit process. Based on the calculation of the IURO for this sample, it is concluded that the average efficiency of the audit committee has increased over the years, which is extremely important given the role of the audit committee which is becoming increasingly important and it is to be expected that in the Republic of Croatia this trend will continue to grow in a positive direction. Furthermore, there was a weak positive but statistically significant correlation between the effectiveness of the audit committee and the type of opinion and the auditor's assessment of going concern, which is met by the assumption that companies with an effective audit committee will have more positive opinions and a positive assessment of going concern while between audit committee and key audit issues there is a weak positive correlation, but it is not statistically significant. Also, it was researched the interdependence between the efficiency of the audit committee and the selection of the Big Four for the audit company and found the existence of weak positive and statistically significant correlation with the average efficiency higher by 10.21 points in companies that chose the Big Four for the audit company.