Object-oriented patterns represent abstractions of good solutions to recurring problems in object-oriented software design. This abstractness means that a given pattern can be used in many different applications, which makes it a valuable tool for constructing reusable software and for helping object modelers achieve more effective results. However, patterns are invariably described informally in the literature, generally using natural language together with some sort of graphical notation, which makes it very difficult to give any meaningful certification of software developed using them. In this paper, we describe a formal model of patterns which can form the basis for demonstrating that a particular design conforms to a given pattern, and we illustrate using an example how this can be done. We also brie y discuss future work which will extend the model to include more behavioral properties.