Models and simulations have always played an important role in engineering and systems engineering. Physical scale models, full-sized models, and computer models are commonly used in all forms of engineering and design. In recent times, interest in modelling has increased to span the full system lifecycle and there has been a significant focus on Model-based Systems Engineering (MBSE). The extension of formal modelling into all phases, and particularly the conceptual design phase, of a system development is a significant step and proponents of MBSE suggest that it will provide considerable benefits. The application of modelling requires considerable care, however. A model, by its nature, is only an abstraction of a real-world domain in which certain parameters have been chosen by the modeller for implementation in the model. Since it is not possible to model all of the parameters of the real world, a model is therefore always an abstraction (deliberately or not) of a real-world domain. The specific nature and level of abstraction mean that a model is only able to serve the purpose for which it is designed-application of the model outside those constraints can be misleading at best and potentially dangerous. In this paper we focus on the use of MBSE to support requirements engineering. We first describe a suitable framework within which to consider the utility of MBSE to support requirements engineering. We then outline the principal activities undertaken as part of requirement engineering and identify the ability of MBSE to support each of those activities. We conclude by identifying a range of challenges that must be addressed before MBSE can be applied usefully to requirements engineering.