Given the rapid rise of model-driven software development methodologies, it is highly desirable that tools be developed to support the use of design patterns in this context. This paper presents such a tool, called LAMBDES-DP, with UML diagrams as the software design models. Its theoretical foundation is a descriptive semantics of UML in first order logic, and the design patterns are formally specified in the same language. The tool uses the LAMBDES system to translate the UML diagrams into their descriptive semantics and invokes the theorem prover SPASS to decide whether the design conforms to a pattern. Our experiments show that our tool has significantly lower rates of false positive and false negative errors compared with existing tools. 1 Motivation and Related Work Software design patterns are frequently used to share design expertise. They document solutions to commonly occurring design problems. It is widely recognized that their proper use can improve software quality and ...