This paper discusses the principles of performance based structural design and motivates the need for probabilistic assessment of the response of structures and an assessment of the consequences of failure. Using the results from a series of tests carried out at SP in Sweden, we extrapolate data required for the assessment of timber structures under a range of parametric fires. This data also includes information required to develop probabilistic models of the response of timber elements under different parametric fires. Using methodologies from the literature, we then carry out a reliability analysis of timber structures, considering uncertainties the timberresponse to fire. This is carried out using the first order reliability method. We show that the opening factor has an influence on the reliability of timber structures, as a result of the rate of heating in a parametric fire exposure. A minimum reliability, evolving over time, is seen to occur at an opening factor of 0.14m1/2. Finally, we propose a modification to the Eurocode target reliability indices that allows these to be used as a target reliability index for structures exposed to fire. The proposed modification is dependent on the floor area and the method is exemplified here for a range of floor areas and its application to timber structures is illustrated.