There has been significant recent interest, within the aspect-oriented software development (AOSD) community, in representing crosscutting concerns at various stages of the software lifecycle. However, most of these efforts have concentrated on the design and implementation phases. The focus of this paper is on representing aspects during requirements modelling. In particular, the issue of how to model aspects as part of scenario-based modelling is addressed. The use of scenarios is common in requirements development and analysis. The authors describe how to represent and compose aspects at the scenario level. Aspectual scenarios are modelled as interaction pattern specifications (IPSs) and are composed with nonaspectual scenarios using instantiation and special composition operators. The composed collection of scenarios can then be translated automatically into a set of state machines using an existing state machine synthesis algorithm. The resulting set of state machines is an executable form of the scenarios and can be used for simulation and analysis of the requirements.