The objective of this research was to develop a functionbased method for analyzing the critical sequences of events that must occur for complex space missions to be successful. The resulting methodology, the Function-based Analysis of Critical Events, or FACE, uses functional and event models to identify the changes in functionality of a system as it transitions between critical mission events. Two examples are presented that detail the application of FACE to prior mission failures including the loss of the Columbia orbiter and the failure of the Mars Polar Lander probe. The result of the research is a methodology that allows designers to not only reduce the occurrence of such failures but also analyze the specific functional causes of the failures when they do occur.