Housing finance and, specifically, the subprime private label securitisation market in the US, was at the epicentre of the global financial crisis. Excessive debt expansion in the run-up to the crisis resulted in credit risk, under-identified and mispriced ex ante, and in systemic risk. This paper considers the role of financial innovation in debt markets and the changing market structure of securitisation in the evolution of the US housing price bubble. New financing vehicles contributed to growing risk, but the more salient factor was the change in the structure of securitisation, which led to unsustainable levels of debt.