Design patterns can be regarded as an approach to encapsulate and reuse good design practices. However, most design patterns are specified using informal text and examples. To obtain all of the benefits of patterns, formal specification and tool support are indispensable. This paper proposes a Design Pattern Specification Language (DPSL) that is both manageable and effective. The DPSL provides software developers with the capability to treat design patterns as concrete design units without lowering abstraction. To demonstrate the usability of our DPSL and its application in design modeling, we have developed a prototype tool that supports the DPSL in UML diagrams. This prototype allows us to demonstrate the tool's support possibilities and the usability of patterns for software development applications.