After creating flows and processes, it is time to create product systems. They are life cycle models and are used to to calculate inventory results and impact assessment.

A "product system" is described by ISO 14040 as a "collection of unit processes with elementary and product flows, performing one or more defined functions, and which models the life cycle of a product." In openLCA a product system is a set of processes connected by flows, performing one or more defined functions and modelling the life cycle of a product. A product system has a reference process with a defined amount of the product (referred to the functional unit), which serves as basis for calculating impacts for all connected processes within the system.