This text synthesizes ten years of research in top-down program design and verification of program correctness. It shows how these techniques may be used in day-to-day programming with the PASCAL language. Many examples of programs and proof development, as well as an explanation of control and data structures, are pro- vided. As a PASCAL programming text, it not only provides a self-contained introduction to the language, but also offers algorithms which operate on sophisticated data structures, and provides the full axiomatic definition of PASCAL in terms of proof rules. Although an introductory course in programmming is required, no particular mathematical background is nec- essary beyond the basic idea of a mathematical proof. A carefully chosen sample of algorithms, including some examples from business data processing, is presented. This collection is supplemented by an extensive set of exercises.