Chapter 2 introduced the basic framework that will be used throughout the book to think about performance issues in computer systems: queuing networks. That chapter concentrated on the qualitative aspects of these models and looked at how a computer system can be mapped into a network of queues. This chapter focuses on the quantitative aspects of these models and introduce the input parameters and performance metrics that can be obtained from the QN models. The notions of service times, arrival rates, service demands, utilization, queue lengths, response time, throughput, and waiting time are discussed here in more precise terms. The chapter also introduces Operational Analysis, a set of basic quantitative relationships between performance quantities.