Abstract. Aspect-orientation has gained increasing popularity, especially within the programming domain, with textual-based approaches such as AspectJ. Aspect-orientation provides an approach to the organization and management of code that cross-cut elements of the base program or library. Cross-cutting aspects is also an issue within the modeling domain, and it is therefore likely that modeling languages can benefit from the aspect-oriented approach. This paper proposes activity aspect diagrams for UML 2 activity models. Activity aspect diagrams are defined directly in the concrete syntax of activity models in order to enable a user-friendly way of specifying aspects. The activity aspect diad base activity models are transformed into the abstract syntax of algebraic graph transformation systems, where the model weaving is carried out using the well-established AGG tool. The approach is demonstrated by two examples and a proof-of-concept aspect diagram editor has been implemented.