Since the emergence of the formal discipline of project management, academics and practitioners have sought to define criteria against which project success can be measured. Perhaps the most well known criteria are encapsulated in the ‘Iron Triangle’that places Cost Time and Quality at the center of project success. However it has been suggested that whilst this triple constraint is important, it can also narrow the focus away from other crucial project success factors. One area that is gaining prominence within the field of project management is the consideration of sustainability principles and there is an increasing understanding of the need to develop methods, tools and techniques to integrate sustainability criteria into the management of projects. This paper presents the results of an empirical study in which project managers were asked to re-draw the traditional Iron Triangle with the inclusion of sustainability. The results of the study indicate that whist sustainability is seen by practitioners as a key factor to be included in project planning and implementation, there is disagreement as to where the issue sits in relation to traditional time, cost, quality constraints and how sustainability principles should be integrated into projects.