Repetitive, abnormal behaviour patterns are performed by tens, perhaps hundreds, of millions of animals worldwide: animals that live on farms and in laboratory animal facilities, stables, kennels, zoos, even in our homes. Our introductory chapter reviews the extent of research into this 'stereotypic behaviour' – traditionally defined as 'repetitive, unvarying, with no obvious goal or function' – since the book's first edition was published in 1993. We illustrate the growing number of papers on captive animals, contrasting them with the much greater number on human clinical subjects and on research animals invasively manipulated to produce abnormal behaviour. We report recent meta-analyses showing patterns in captive animals' stereotypic behaviour (e.g. systematic variation with taxon, and relationships with other signs of poor welfare). We also review the outstanding research questions raised by the first edition, and how they have been tackled in this one. Some contributed 'boxes' in this chapter further set the scene for the rest of the book: one summarises the motivational hypotheses typically advanced by ethologists, a second introduces explanations in terms of brain (dys)function, a third reviews how ‘coping’ might help account for stereotypic behaviours, while the last considers what is meant if they are labeled ‘pathological’.