Product modeling is the specification of the product variety and also comprises the necessary reasoning, the user interface, interfaces to other business processes, IT, etc. It relies on a set of generic concepts like part trees, type hierarchy, feature values, table constraints, etc., to distinguish the instance (application) and model (domain) layers. This chapter focuses on theoretical and practical aspects of representing product configuration problems effectively. It introduces three layers of representation in product configuration, provides details on several aspects important in modeling configurable products, and introduces an UML-based modeling notation.