The paper focuses on the importance of an early engagement of stakeholders in order to manage the project along its entire life cycle. An increasing level of complexity tends to generate an increasing level of unpredictability, since it is difficult to anticipate all the possible dynamics in a complex project. Improving the forecasting/planning process requires using all the knowledge available to the project team, in particular when facing a high level of project complexity. In fact, stakeholders are the main sources of knowledge about the project and their early engagement may significantly increase the amount of knowledge available both for project planning and control. As a consequence, project planning may be considered as resulting from the interaction of the project team with all the stakeholders involved in the project.