The National Infrastructure Simulation and Analysis Center (NISAC) is responsible for providing fundamentally new modeling and simulation capabilities for the analysis of critical infrastructures. We believe that by developing and exercising a variety of modeling approaches, over time we can build up a mosaic of collected insights and intuition and understanding about how complex, interdependent infrastructure systems will respond to disruptive events. This article describes some of our modeling efforts to date as related to energy and telecommunications and their interactions. We consider highly-abstracted models that allow experimentation about the interplay between network topology and tolerance for evaluating system robustness against cascading failures. And at the opposite end of the spectrum, we also consider high-fidelity models that include a sufficiently realistic representation of infrastructure elements to allow simulation of infrastructure responses to a variety of disruption scenarios. We conclude this article by showing how systems knowledge accumulated from this range of modeling efforts was used to forecast the infrastructure impacts from Hurricane Katrina.