As a result of the powerfully increased international fire research during the last decades, more and more components and systems within fire safety engineering are becoming amenable to analytical and computer modeling. The most manifest verification of this development probably holds for the fire engineering design of load bearing and separating structures. The paper-begins with a general survey on the characteristics of a reliability based structural fire engineering design. Then, the physical model, describing the simulated real fire exposure and the thermal and mechanical behavior of the structure, is commented on with regard to the present state of knowledge and the need of further research. Finally, the different uncertainty sources in the design process and the principles of a cost-benefit optimization are dealt with and exemplified.