This chapter addresses domain modeling. IFML does not prescribe a specific domain modelling language but can be interfaced to any notation preferred that expresses the objects and associations of the application domain. The chapter employs UML class diagrams, and briefly recaps their main features for structural modeling. It discusses design patterns for the domain model, which stem from the joint consideration of data representation and interaction support requirements. The chapter ends with the specification of the domain model of an e-mail application.