Calculation of the top event probability plays a central role in quantitative fault tree analysis, as it serves as a basis for many other dependability metrics, such as the availability, reliability, and component importance. This value can be computed exactly using the techniques from the next chapter. However, for large fault trees, the exact calculation can be infeasible. In such cases, the top event probability can be efficiently approximated using the minimal cut set list, calculating the probability of each cut set. This chapter outlines three of these methods, detailing their assumptions and precision.