The thesis discusses how, when and why Client introduces the work of an Engineer, who carries out his duties under the rules of FIDIC contract. The first part presents the construction project and its complexity, as well as a detailed definition of participants, who are required by the Construction Act to participate in the process. From the latter, it is evident that the duties and powers, as obligation of supervision, of the Technical Supervisor to a large extent overlap with the ones of an Engineer. Therefore, the thesis focuses on the type of contract between an Employer and a Technical Supervisor, as well as the powers received by the Technical Supervisor at the conclusion of different types of contracts. I am also pointing out Engineer’s powers and duties arising from the FIDIC Red Book. Based on the list of the powers necessary to carry out supervision, extended supervision and engineering regarding FIDIC contract, the Employer’s decision on what kind of contract will be concluded, will be easier.