Extreme weather events, such as the UK floods of 2007 and cold snap of 2010–2011, stress the importance of infrastructure systems' resilience for business continuity. The interconnected nature of critical national infrastructure and its component parts places demands on the approach used to deal with its subsequent complexity. Recognition of infrastructure as a complex adaptive system has led to the development of an innovative, systems-based methodology for sustainability assessment in the built environment. The methodology consists of a database of causal interactions which, when combined with a process, allows users to produce causal loop diagrams that identify unanticipated systemic behaviour, communicate risks, share knowledge, and identify systemic intervention points that minimise negative consequences and add value in a project context. The approach is applied to highlight the key characteristics of complex adaptive systems that critical national infrastructure exhibits and show how the technique can be used to increase infrastructure resilience and sustainability.