It is widely accepted that the key to successfully developing a system is to produce a thorough system specification and design. This task requires a suitable specification language, satisfying the following needs:

• a well-defined set of concepts
• unambiguous, clear, precise, and concise specifications
• a thorough and accurate basis for analyzing specifications
• a basis for determining whether or not an implementation conforms to the specifications
• a basis for determining the consistency of specifications
• computer support for generating applications without the need for the traditional coding phase

SDL has been defined to meet these demands. It is a graphical specification language that is both formal and object-oriented. The language is able to describe the structure, behavior, and data of real-time and distributed communicating systems with a mathematical rigor that eliminates ambiguities and guarantees system integrity. It has a graphic syntax that is extremely intuitive. Even nonconstructors quickly obtain an overview of a system's structure and behavior. The most important characteristic of SDL is its formality. The semantics behind each symbol and concept are precisely defined. Above all, the great strength of SDL lies in describing large real-time systems.