Reputation systems are complex social systems that continually collect, aggregate, and distribute feedback about a person, an organization, a scholarly work, or some other entity, based on the assessments of others from their interactions or experiences with the entity. A reference model, or meta-specification, is presented that describes the essential functionality and behavior that embedded information systems need to provide to be effective and useful mechanisms for making reputation explicit and measurable. The reference model, based on a multi-disciplinary understanding of reputation, is specified in the Unified Modeling Language (UML) since it is the industry-standard approach for modeling systems. The model addresses systems that focus on the reputation of individuals (personal reputation). Extensions for the reputation of corporate, not-for-profit, and governmental organizations (organizational reputation) are described in the conclusion along with challenges for future research.