The MOF Query, View and Transformation RFP, issued by OMG will result in a key enabling technology for modeldriven development of large distributed systems. We have designed a transformation language which will meet the requirements of this RFP, and several others besides. The language is declarative and patterns based. Transformation descriptions are explicitly reusable and modular. Rules that make up such descriptions may be aspect-driven, allowing for transformations to be written to address semantic concepts rather than structural features. This paper introduces the language and its rationale, and shows how it is used to solve a small but non-trivial MDA problem.