Construction projects tend to be more complex with the development of technologies and increase in number of requirements by investors. Having in mind the fact that investors in construction projects increasingly transfer the liability for their implementation to the contractor, it is clear that the majority of impact of the project complexity for its success is borne by the contractor. When speaking of civil engineering and construction projects, as of broad social endeavor which involve a significant number of participants during their implementation, and the result of which a large number of users depends on, based on what has been previously stated, it can be simply concluded that construction projects are complex. For all of this, an adequate understanding of the complexity of the project and its impact on the construction project is a step forward to success of the construction projects. This doctoral thesis defines a framework for assessing the impact of complexity elements on the initiation of risks on the project and thus on the success of the construction project from the point of view of a contractor. For the purpose of establishing the theoretical basis for conducting the research, there has been created a detailed overview of literature on the complexity. The literature overview analyzed the present studies in relation to complexity of the project, and analyzed the literature dealing with the complexity of the construction project from the perspective of contractor. The literature overview puts in focus the most significant definitions of the project's complexity, and there was identified that there is no generally accepted definition of the project complexity. Additionally, it has been determined that the project complexity differs with regard to perspectives of the participants in its realization. The detailed analysis of the literature determined that so far studies did not identify a framework for assessment of complexity elements, observed from the perspective of the contractor. The literature overview explored the relationship between complexity and risk, project success, as well as trust, complexity and project success, all this from the perspective of the contractor. Besides, there has been analyzed the impact of the level of project complexity to activation of risks as well as to results and success of the project. Through the literature analysis, 267 different complexity elements were allocated, as well as 15 groups of complexity. The complexity elements were structured into eight complexity groups, so, scope complexity, organizational complexity, operational and technological complexity, environment complexity, complexity related to resources on the project, legal and socio-political complexity, communication and economic complexity. Each of the stated groups contains ten associated elements of complexity. Groups of complexity and key complexity elements are in the focus considering the frequency of their appearance in existing research as well as their applicability from the contractor’s point of view. Afterwards, the classification of the key complexity elements was carried out through research among construction project managers. The research concerned was conducted among forty-one experts in the area of construction project management from a point of view of the contractor. Beside the classification of the key complexity elements of the construction project from the perspective of the contractor, this part of the research has also analyzed the impact of complexity on the project’s success as well as the impact of trust on the complexity and project’s success, all through the five stages of the works performance in the construction project. Based on the results of the research, graphics of change in the intensity of the influence of the key complexity elements on the success of the project were prepared. The role of trust increases with the size and complexity of the construction project, taking into consideration that along with the size and complexity of the project increase, the number of interactions between project participants also increases. For that reason, the trust among all parties in the project gets more and more important with the increase of the project’s complexity. The success of any construction project would be questionable without trust among the participants in the project, even when there are clearly defined systems of control including the high quality of contractual documentation. When analyzing the research results, one could say that, in most part, the curve of trust impact on the project is in line with the curve of the level of project’s complexity. In accordance to the above stated, one can assert the relation of complexity and success of the project when observing from the perspective of impact of contractor’s trust on the project. The trust decrease can negatively affect the course of the project and produce big losses for the contractor. The trust as a determinant has the most significant impact on the success and complexity of the project in the works implementation phase. The most intensive impact on the success and the complexity of the project has the very trust between the contractor’s project leader and investor’s project leader, as well as between the contractor’s and investor’s companies. By analyzing the results obtained from research using the ANP method in the software programme Super Decisions, a framework was defined for the assessment of complexity elements and the order of key elements of complexity through a particular phase of theconstruction project. From the established framework, the three most significant groups of complexity stand out: • communication complexity • complexity related to resources on the project • environment complexity Communication complexity as a driving force of risks related to project communications as well as determinant of the success of the construction project happens to be frequently subject of the various analyses. Quality communication in a construction project characterized by a significant quantity of interpersonal and interorganizational relations can cause increase of the level of interpersonal and interorganizational trust. To that effect, due to recognition of the importance of communication complexity at the very beginning of this research, this research was extended to the topic of trust. Therefore, for the contractors in a construction project, it is of a crucial importance to manage project communication properly as of the very project beginning, to acquire investor’s and supervising engineer’s trust and thus contributes to a simpler and more favorable implementation of construction project works. Under the analysis of the research results, there stand out the following elements as the most important complexity elements: • Diversity and availability of workforce • Incorrectly planned geological conditions • Communication with the supervising engineer and investor • Concealment of information between participants • The experience of the project manager acquired on similar projects By using the already established order, based on characteristics of his project, design details as well as needs of his company, the contractor at the beginning of every project can allocate key indicators of the project complexity. Thereby, in the initial phases of the project, contractor can prepare proper answers to possible risks activation under impact of such complexity indicators. In this manner, there has been created a basis under which the contactor may at early stage recognize risks, but also to define realistically the project goals. Hence, the contractor, when applying the research results in the planning phase of the project, may affect the mexecution of results in the project as well as the project’s success. Beside to what has been stated above, the impact of the shape of trust on success was analyzed as well as complexity of construction project for the contractor. Additionally, the impact of the form of trust on success and complexity of the construction project among contractors was analyzed. From the already established order of forms of trust, according to their impact on the complexity and success of the project, the three most significant forms of trust stand out, from the perspective of the contractor: • Trust among the members of the contractor’s project team and investor’s project team • Trust between the contractor’s project manager and investor’s project manager • Trust between the contractor’s and investor’s companies Besides to what has been stated above, for each phase of the construction project, the order of the key elements of complexity is defined with regard to their impact on the overall level of complexity of the project, as well as the order of the form of trust according to the intensity of the impact on the complexity and success of the project. After identification of the framework for new survey that had been conducted among thirty-four project leaders regarding implementation of construction projects, the verification of the framework for assessment of the complexity elements was carried out with regard to their impact on risks initiation but also on project’s success. Furthermore, verification of the key complexity elements orders applicability was carried out not only for the whole project but also for specific phases of works performance in a construction project from the point of view of contractor. It was determined that the suggested complexity elements represent driving risks in a construction project. Based on displayed research results, it was confirmed that the contractor can, through recognizing and defining key complexity elements in the project’s planning phase, enable early detection of risks as well as realistic goals setting for the project’s success. The proposed framework, as well as the order of the key complexity elements, has certain restrictions in its practical application, having in mind the variety of construction projects carried out, which certainly can be the subject of future research. Additionally, the continuation of the research should be directed to quantification of the impact of the project’s complexity on the contractor’s unit prices. Thereby, the contractor would achieve a crucial benefit in a more regular entering of the non-linear characteristics of the project in the costs of the works to be performed by him.