Acknowledgments I could not have written this book without the steadfast support and encouragement of my wife Isabel who was there for me, even when I was too distracted to be there for her. Writing a technical book in one's spare time while holding down a consulting career makes one a difficult person to live with. I also owe a debt of gratitude to Geoff Howard and Karen Richards of Ajilon, who have allowed me to work part-time so as to be able to put the time into this book that it required. During my career I have had the good fortune to meet and have my imagination stimulated by a variety of thinkers. Foremost among these is Graeme Simsion, who not only launched my consult- ing career by inviting me to join Simsion Bowles and Associates, but also trusted me to co-author the second and third editions of his seminal book Data Modeling Essentials. Others from whom I have learnt many of the insights that have made their way into this book include Geoff Bowles, Chris Date, Harry Ellis, Terry Halpin, David Hay, Ron Ross, Hu Schroor, Alec Sharp, and Michael Stonebraker. More recently, the opportunity to work in a rules-rich environment alongside Colin Campbell of New South Wales Land & Property Information, and my Ajilon colleagues Greg Bowen, Emma Craig, and Pia Nicod, has enabled me to put many of the ideas in this book to the test. Although this book is aimed at system design professionals, it is as much as anything a book about language. My love of language was initially stimulated by my late grandfather Jack Cade, who gave me copies of Frederick Bodmer's Loom of Language and Roget's Thesaurus during my teens. I have also been privileged to have had some wonderful language teachers over the years: Cor- nelius O'Neill, who taught me a year's worth of Latin over the course of my first Australian summer ´ so that I could join the university stream at High School; Zorana Popovic and Liljana Tasevska, who