Fifty years ago, most economists confidently predicted that mergers would generally boost financial performance. They would bring scale economies and market power; or they would reinvigorate sleepy targets. Examples are given of successful deals which have indeed delivered such outcomes. However, in the subsequent fifty years many statistical studies cited in Appendix 1 have found that most mergers were zero or negative sum: an eminent consultancy concluded that ‘roughly 70% fail’. The mystery in the book’s title is that as this evidence of disappointing M&A results has mounted, executives have doubled down on acquisition activity – globally now totaling forty times the annual number of deals seen forty years ago. Before outlining the plan of the book the chapter delves deeper into what constitutes ‘success’ or ‘failure’.