Sustainability is trending. With large numbers, architects are part of the movement that is in search of a more sustainable world. An important part of this search is the urge to prove sustainability. Currently available methods to prove sustainability in architecture play into this demand. However, practice shows that these methods are not always well received as professionals experience issues and inconsistencies with them. How is it, that proving sustainability is so difficult, that even these great corporations cannot deliver a method to unequivocally prove sustainability? This questions is researched in this study. Professionals in the world of (Dutch) sustainable architecture are interviewed. Analysis of these interviews showed that the problem of proving sustainability is such an immensely complex and extensive one, that it can – and should – be approached as a Wicked Problem. Theory on Wicked Problems and results from the interviews are combined to explain the difficulties one faces when attempting to prove sustainability in architecture. Literature on strategies to approach Wicked Problems is discussed, to give an understanding on how problems of this size can be tackled. This gives a better understanding of why proving sustainability cannot be done with one general method, but rather needs a multitude of methods or approaches as every project is unique and therefore actually needs a tailor-made method of proof. Several approaches on how to think about proving sustainability are mentioned that might pique the interest of the architect.