Configurable products have customizable parts (components) and properties (features) with rules (constraints) to ensure technical feasibility and quality. They come in many kinds: physical (hardware) or intangible (software, services), small (e.g., pen) or large (e.g., power plant), simple (e.g., pizza) or complicated (e.g., production schedule), components (e.g., power supply) or systems (e.g., railway interlocking and control), for consumers (e.g., mountain bike) or business (e.g., container ship). Configurators are useful throughout the whole product lifecycle, from pre-sales to service, and have different focus areas in different phases. This chapter defines the meaning of the terms product and configuration. It shows how variety can be specified by means of concepts such as part-of hierarchy and kind-of hierarchy in a configurable industrial product and an extension of our folding bike example. And it gives an overview over various kinds of configurable products and over different classes of configuration systems.