Modelling is often used as a tool to help with design, analysis, and management of systems. With systems growing in complexity, with diverse components and interconnections with other systems, modelling approaches can benefit from an understanding of interfaces—both how they can be represented in models, and how they be used between models. In this work we discuss this dual view of interfaces, and develop a set of four elements of interfaces that modelling approaches should be able to capture: functionality, specification, policy, and environment. We then look at how this view of interfaces can be implemented, using an extension to an existing modelling framework as an example.