I discuss formal description techniques (FDTs) as they are applied in practice in software and system engineering. Their quality can be measured by their formality, descriptiveness and technical usefulness. I discuss shortcomings in the scientific and semantic foundations of FDTs. I formulate requirements for FDTs. I demonstrate how a family of complementary description techniques can be developed on the basis of a mathematical system model that provides a scientific methodological foundation for the modular specification and development of systems. This demonstrates a proceeding in the design of FDTs that helps to make them formal, descriptive and technically useful. Keyword Formal description techniques, mathematical semantics, mathematical system model