In this chapter, we consider workloads in the context of workload generation. It starts with a classification of the different workload facets and artifacts. We introduce the distinction between executable and non-executable parts of a workload, as well as the distinction between natural and artificial workloads. The executable parts are then discussed in detail, including natural benchmarks, application workloads, and synthetic workloads. Next, the non-executable parts are discussed, distinguishing between workload traces and workload descriptions. In the rest of the chapter, we introduce the different types of workload descriptions that can be used for batch workloads and transactional workloads as well as for open and closed workloads. The challenges of generating steady-state workloads and workloads with varying arrival rates are discussed. Finally, the chapter concludes with a brief introduction of system-metric-based workload descriptions.