This chapter describes the transition from a mental image of the system to a conceptual model that captures the structure of the system and its behavior. It introduces the single-server queueing system. A conceptual model is a representation of a system using specialized concepts and terms. A mental image reflects how the modeler perceives the system and its operation. The mental image should include only those aspects of the system that are necessary for the simulation study. A Discrete-Event Systems (DES) is made up of elements referred to as entities. The meaning of the name "discrete-event simulation" is not clear for many people. The word "event" indicates that the simulation is advanced by the occurrence of events. An entity is characterized using attributes, which are local variables defined inside the entity. The coffee machine is a static entity because it does not move in the system and its purpose is to provide service only for other entities.