A description of infrastructure is—the basic physical and organizational structures and facilities (e.g., buildings, roads, and power supplies) needed for the operation of a society or enterprise. In other words, a platform (or boat) that enables us to regularly go to places we could not otherwise visit, and do things we have not done before. In this chapter, we take organizational structures to include financial, informational, and knowledge components, and consider their combination with physical components. Infrastructure or its use may be invisible to us until it is disrupted in some way, and we pursue the theme of the need for absolute reliability. Infrastructure of any kind may be viewed as a system of systems, with its vulnerabilities associated with the weakest link or coincident failures of some sort in more than one subsystem. The ability to optimize infrastructure operations, test for rare events, and train to respond to them leads us to discuss the application of models and simulation tools.