In this paper we present a new methodology for modelling the development of the prices of defaultable zero coupon bonds that is inspired by the Heath-Jarrow-Morton (HJM) [19] approach to risk-free interest rate modelling. Instead of precisely specifying the mechanism that triggers the default we concentrate on modelling the development of the term structure of the defaultable bonds and give conditions under which these dynamics are arbitrage-free. These conditions are a drift restriction ...