Some years ago a book reviewer, perhaps with Freudian honesty, remarked that the book in question 'filled a much needed gap in the literature'. That phrase has haunted the writing of this gap-filler and this preface may be considered an apologia. For a number of years I have found myself teaching various groups of students about cell locomotion and cell behaviour: sometimes science students specializing in cell or molecular biology, sometimes immunologists or pathologists who only wanted a broad background introduction. Those students who were enthusiastic, or who wished to appear so, asked...