Design Patterns are now widely accepted as a useful concept for guiding and documenting the design of object-oriented software systems. Still the UML is ill-equipped for precisely representing design patterns. It is true that some graphical annotations related to parameterized collaborations can be drawn on a UML model, but even the most classical GoF patterns, such as Observer, Composite or Visitor cannot be modeled precisely this way. We thus propose a minimal set of modi - cations to the UML 1.3 meta-model to make it possible to model design patterns and represent their occurrences in UML, opening the way for some automatic processing of pattern applications within CASE tools. We illustrate our proposal by showing how the Visitor and Observer patterns can be precisely modeled and combined together using our UMLAUT tool. We conclude on the generality of our approach, as well as its perspectives in the context of the de nition of UML 2.0.