Understanding complexity is of significance importance for project managers because of the differences associated with decision making and goal attainment that appear to be related to complex projects. As projects have become more and more complex there has been an increasing concern about the concept of project complexity as it influences upon the project management process. The importance of complexity to the project management process is widely acknowledged for several reasons. Complexity affects the modelling, evaluation, and control of projects and the objectives of time, cost, quality and safety. Complexity can also affect the selection of an appropriate project organization form and the project management arrangement including the expertise and experience requirements of project managers. In this paper the concept of project complexity is analysed and the main models of project complexity are presented. Our results evidence two aspects, the lack of consensus when defining project complexity and the fact that the focus of complexity models is basically on goals and methods, the level of agreement and certainty or the number of elements and their interdependency no taking into account other critical aspects of complexity.