The design of this book is based on teaching the ]SP (Jackson Structured Programming) methodology to undergaduates and postgraduates over a period of a number of years. I am grateful for the comments and feedback that have been provided by students who have taken these courses. The aim of the book is to provide readers with an understanding of the concepts behind the ]SP methodology in order that they may apply it for themselves; simply using the notation is not sufficient, it must be used appropriately. The answer to the question "Why is this wrong?" can lead to a greater understanding than a sim­ ple response to "Is this right?". I have included illegal structures as "understandable mistakes" in the early sections for this reason. It is not necessary for readers of this text to have experience with any particular programming language; indeed, one of the virtues of ]SP is that it is language independent. Examples have been given in Pascal, C and COBOL as these are languages which students of ]SP are likely to have met in the course of their studies, or will be meeting while they are learning ]SP. The COBOL language is widely used in industry in a ]SP development environment.