Offering a tutorial introduction to the VDM formal notation, this text is designed for second- and third-year undergraduates of software engineering. Using notation that blends together both computing and mathematics, it covers the process of writing formal specification, developing abstract modules of data and operation, and design. Special attention is paid to the difficult topic of operation decomposition. The authors describe specific proof obligations, enabling students to develop their software so that the conformity of the design to the specification, and the program code to the design, is guaranteed. Teaching aims are highlighted at the start of each chapter, and the text is based on the Open University software engineering degree.