We propose a figure of merit that characterizes the femtosecond laser damage behavior of optical coatings. This figure of merit, based on the complete spatiotemporal evolution of the field in a multilayer system, can be included in optics design. The monochromatic intensity enhancement widely used in “electric field-engineering” is sufficient only in certain structures such as high-reflectivity quarter-wave mirrors. In more complex systems, for example, in group delay dispersion mirrors and frequency tripling mirrors, one should consider the actual (typically smaller) intensity enhancement produced by short pulses and the change (typically increase) of pulse duration within the stack.