Russell Ackoff's long and distinguished career as the doyen of Design and Systems Thinking was built around a collection of deceptively simple - but often overlooked - principles and observations. In "Differences That Make a Difference" - the last of his many books - Ackoff determined to distill the wisdom of a lifetime into a 'glossary' that would be easily accessible to managers, employees, students, and academics alike. His aim was to dissolve (not solve or resolve) some of the many disputes in professional and private life that revolve around meaning and (mis)understanding. For example, development and growth do not mean the same thing. A cemetery or rubbish heap can grow without developing, whereas a person continues to develop long after he or she has stopped growing. Ackoff understood that getting to the bottom of differences like this one could have far-reaching practical consequences for improving our organizational health. In "Differences That Make a Difference", he has succeeded magnificently in creating what Charles Handy in his Foreword calls 'a manual for clear thinking'. And if the world ever needed clear thinking...