The modelling of default risk in debt securities involves making assumptions about the stochastic process driv- ing default, the process generating the write-down in default, and risk-free interest rates. Three generic approaches have been used. The first relies on modelling the value of the assets on which the debt is written. The second involves modelling default as an arrival process. The third involves directly modelling the interest rate spreads to which default gives rise. Each of these approaches may be applied to the impact of default risk on derivative products such as swaps and options. One application is to the valuation of derivative products that may default. The other is to the new class of 'credit derivatives' that represent derivative products written on credit risk.