Here are the original slides and video from my first lecture at the LSE complexity programme, exploring the underlying mechanisms of phase transition and non-linear change in social systems. This talk focused heavily on the underlying mechanisms of collapse and non-linear transitions. The evidence suggests that systems become vulnerable to phase transition (i.e., collapse) when they experience exponential growth, increased connectivity, decreased reserves, and increased imitative behaviour. Put simply, too much connectivity, too much interactivity and too little resilience means even a tiny change can lead to massive fluctuation and collapse. I introduce and explore the work of Charles Perrow (Normal Accident Theory), Didier Sornette (Why Stock Markets Crash), Joseph Tainter (The Collapse of Complex Civilisations) and Gunderson and Holling (The Adaptive Change Cycle), showing how each contributes to a greater understanding of how, when and why collapse occurs. Take home message? Collapse is endemic to many classes of complex adaptive systems, they tend to occur in certain regular ways, and we can use this knowledge to understand and prepare for radical change.