Projects are implemented in many activities, such as execution of works, provision of services, and in a broader sense include a number of business activities. They are extremely important because successful project management ensures the realization of many complex goals. However, despite such and so much interest in projects, researchers and business people believe that over 50% of launched projects end in failure. In addition to failure, projects are also marked by crises, risks, insecurity and incompetence. Despite such results, in recent years there has been a projection of the economy and society as a whole, both globally and nationally. Following this, it should be emphasized that one of the most important decisions in project management is the selection of project managers, which is made after the selection process, which should be based on the model of competence. One of the highly ranked criteria in selecting a project manager is the possession of a project management certificate, where certification is often equated with proof of project manager competence. Previous research has shown that project manager certification affects project success. The subject of this dissertation is the research of the impact of project certification on project success, where certification is measured through the possession of certificates, and success through the efficiency and effectiveness of projects. The research is also focused on the impact of project complexity on the mentioned relationship, all with the aim of increasing the success of the project. In order to investigate this issue, an empirical study was conducted on a sample of project managers from the electricity sector. Theoretical and empirical research has achieved the set research goals and confirmed the research hypotheses: (1) The certification of the project manager has a positive impact on the success of the project in the electricity sector of the Republic of Croatia. (2) The impact of the project manager's certification on the success of the project depends on the complexity of the project in the electricity sector of the Republic of Croatia. (3) There is a difference in the assessment of the importance of the project success criteria between certified and non-certified project managers in the electricity sector of the Republic of Croatia. The scientific contribution of the research can be seen in the originality of the proposed research model whose empirical testing has resulted in the development of new scientific knowledge about the certification of project managers, the complexity and success of projects. In theoretical terms, the contribution is manifested in the creation of a general theoretical framework of the impact of project manager certification on project success as a result of combining the current most significant theoretical settings of the relevant scientific literature. In terms of application, the most important contribution is the validation of the model of the impact of project manager certification on project success in the electricity sector of the Republic of Croatia. A post hoc analysis of the impact of the characteristics of the leader and the project on the success of the project was made, the limitations of this research were stated and recommendations for further research were given.