This chapter is about observing the behavior of systems. Systems can be thought of as comprised of two parts, a static or structural part, and a dynamic or behavioral part. For example, consider an equity trading market. There is a physical location where the market resides (even “virtual” markets require some form of infrastructure to house the network servers and support personnel), and the operations that are conducted to perform transactions. The static part of a system is usually more accessible than the dynamic part, because the dynamic part is based on system operation-requiring long-term observation to understand all of the interactions. Thus, discovery of system behavior is often the more challenging task.