Infrastructure projects are complex and technical complexity is dwarfed when compared with the social complexity caused by the interaction of individuals and organisations that participate and influence these projects. During conception and planning, uncertainty and conflicting motivations influence decisions that drive the solution and plans that are carried out during construction, operation and finally disposal of the system. If the need is not accurately conveyed and the solution is not properly defined, estimated and planned, infrastructure projects become subject of costs overrun and schedule delays of serious consequences, including the possibility that they may not satisfy the real need. Understanding the sources of uncertainty, what drives motivation, how private and public organisations and social groups influence each other is of great importance for planning and managing successful infrastructure projects. Although it is difficult, if at all possible, to quantify the influence that individuals and social organisations may have over the project, principles and insights taken from social and systems theories can help to create qualitative mental models that help in dealing with the inherent complexity of these large projects. This paper compiles principles and insights extracted from systems and social theories into a simple framework that can be applied by infrastructure planners and decision makers to better cope with complexity, development and managing projects with better chances of success. The paper concludes suggesting how modelling and simulation can be used to assess 'what-if scenarios' that would help in developing a better understanding of the consequences of social interaction in the success or failure of infrastructure projects.