The research problem of the doctoral thesis is focused on examining the influence of institutional investors on corporate governance of companies in the Republic of Croatia. Previous research confirms the thesis that the presence of institutional investors in the ownership structure of joint stock companies positively affects the value of companies by limiting the actions of company management to act solely for their benefit, thus reducing the gap between ownership intentions and business operations, which results in increased efficiency and business security. The problem was approached comprehensively, resulting in a fuller insight into the impact of institutional investors on the corporate governance of companies in their portfolio. Institutional investors were analysed as homogeneous but also as heterogeneous groups (investment funds, pension funds, insurance companies), which has eliminated the problem of inclusivity of the results of the analyses so far, in which investors were viewed exclusively as homogeneous groups. The paper empirically tests the significance of institutional investor ownership influence on corporate performance and safety and examines the views of institutional investment management company managers on the level of involvement in the management process with the aim of identifying guidelines for improving and increasing the control functions of institutional investors in corporate governance of the companies. The research of this doctoral thesis presupposes a combination of both cabinet and field type, since data from secondary and primary sources will be collected for the defined research problem. The empirical research was conducted on the basis of secondary data obtained from the databases of the Central Depository and Clearing Company (CDCC), Zagreb Stock Exchange (ZSE), Croatian Financial Services Supervisory Agency (HANFA) and individual analysis of the semi-annual reports of institutional investor management companies included in the research. The survey sample consists of 55 joint stock companies with headquarters in the Republic of Croatia according to the following criteria: (1) the issuers of shares are non-financial corporations, (2) reference is made to ordinary shares, (3) the companies continue to operate in the analysed period of time from 2010 to 2016, (4) these are the most liquid shares on the Croatian regular market, (5) the companies whose ownership structure include institutional investors (investment funds, pension funds, insurance companies). As with the selection of companies, the analysis of institutional investors likewise, a balanced sample was used, where investment funds, pension funds and insurance companies operating continuously from January 2010 to December 2016 were selected, whose portfolio clearly shows investment in ordinary shares on the Croatian capital market, in those companies that were involved in empirical studies of the impact of institutional investor ownership on corporate governance in order to obtain a holistic approach to corporate governance analysis of the companies selected. Therefore, the target sample of this research includes 10 investment fund management companies, 4 pension fund management companies and 17 insurance companies. The overall objective of this research is to describe, identify and examine the impact of institutional investors on corporate governance of the companies in the Republic of Croatia. Based on the literature researched so far and the general scientific objective of the research, it is possible to define specific research goals by looking at institutional investors as homogeneous and heterogeneous groups: SC1: Classify, compile and describe institutional investors as a whole, their individual types, their controlling role, and describe, identify and examine the area of corporate governance and the nature of the impact on corporate success and safety based on empirical and theoretical research to date; SC2: Classify, compile and describe past research into the impact of institutional investors on corporate governance in portfolio companies by analysing institutional investor management characteristics, and depending on the type, period of the investment time, and level of involvement in the corporate governance process; SC3: Based on the results of the research, define guidelines for improving and increasing the control of institutional investor management in corporate governance in the Republic of Croatia with the aim of increasing efficiency and security. In accordance with the defined research problem and the research objectives set on the basis of it, the following basic and supporting hypotheses have been derived: H1: Ownership of institutional investors has a significant positive impact on the success and safety of companies in the Republic of Croatia. H1.1.: Ownership of institutional investors viewed as a homogeneous and heterogeneous group has a significant positive impact on the success and safety of companies in the Republic of Croatia. H1.2.: There is a linear influence of the ownership share of institutional investors viewed as a homogeneous and heterogeneous group on the success and safety of companies in the Republic of Croatia. H2: Institutional investors are involved in the corporate governance of companies in their portfolio. H2.1.: Active participation in the decision-making process is achieved through voting rights at the General Assembly, communication with the Management Board and cooperation with other institutional investors present in the ownership structure. H2.2.: Institutional investors prefer a long-term horizon of investing in liquid companies with clearly defined strategic developments, continuous and small increases in stock prices, dividend payments and low investment risk. H2.3.: Comprehensive investment legislation limits the activities of institutional investors in the corporate governance of companies in their portfolio. For the empirical part of the research on the impact of institutional investor ownership viewed as a homogeneous and heterogeneous group (pension funds; investment funds; insurance companies) on the success and safety of enterprises in the Republic of Croatia, panel data or longitudinal variables containing both spatial and temporal dimensions were used, specifically static and dynamic panel models. The spatial dimension is represented by a sample of 55 Croatian companies, while the time dimension of the research is focused on semi-annual observations from the time horizon from 2010 to 2016. In order to test the second hypothesis, a test method, a specifically structured interview, was used in the research, and descriptive statistics were used to process the “closed” type of question and the reliability of measuring scale used was tested by Cronbach's alpha coefficient. Content analysis, specifically Conceptual analysis, was used to analyse the “open” type of question. The text has been coded into categories based on selective reduction, to ultimately consist of words, sets of words, or phrases that are indicative of the analysis. The results of the analysis show that the institutional investors are involved in the corporate governance of the companies in their portfolio, and the control mechanisms they use include voting rights at the general meeting which gives them explicit power in the fundamentals of corporate governance control, direct communication with management to discuss the strategy of the future enterprise development and cooperation with other institutional investors present in the ownership structure of each individual company. Consequently, a statistically significant impact of institutional investor ownership viewed as a homogeneous and heterogeneous group on the success and safety of the business operations of the companies in their portfolio has been identified, which reduces the gap between ownership intentions and management interests, resulting in an increase in shareholder wealth while limiting satisfaction and social goals of a company.