Design patterns encode proven solutions to recurring design problems. To use a design pattern properly, we need to 1) understand the design problem the pattern resolves, 2) recognize an instance of this problem in the model at hand, and 3) to transform the model to produce the proposed solution. We argue that an explicit representation of the design problem solved by a pattern is key to supporting each one of these tasks. We propose to represent a design pattern using a triple (MP, MS, T) where MP is a model of the design problem solved by the pattern, MS is a model of the solution proposed by it, and T is a rule-based representation of the transformations embodied in the application of the pattern. In this paper, we describe the principles underlying our approach and the current implementation using the Eclipse Modeling Framework and JRules.