This chapter shows the reader how the transition from a system description to a simulation model is made. It presents three case studies. The first case study is about estimating the reliability of a network using the Monte Carlo methods and several variance-reduction techniques. The second case study is about modeling a point-to-point wireless transmission system where packets may be lost either due to a full queue or bad channel state. The final case study is about modeling a simple error-control protocol and studying the impact of error probability on system throughput. A network is referred to as static if time plays no role in its model. A network can be modeled as a graph which consists of vertices and edges. Two measures of performance are considered: average delay and percentage of successfully received packets. Automatic Repeat Request (ARQ) is an error-control technique used in computer networks. It is based on the use of acknowledgment messages and timeout interrupts.