This book is about a system development method. The method, known by the acronym JSD, is a method for specifying and implementing computer systems. We use the general term 'development' to cover a range of activities usually carried out by people whose job titles are 'systems analyst', 'system designer', 'program designer' or 'programmer'. These activities include requirements specification, functional specification, logical system design, application system design, physical system design, progr am specification and design, program implementation, and system and program maintenance. The book is addressed to people, whatever their job titles, who engage in any of these activities, or want to understand how they may be approached.