More than two thirds of acquisitions fail to create meaningful shareholder value. When mergers between similar companies seem an obvious way of gaining opportunities of scale and scope, why do so many lead to disaster? Richard Carr, Graham Elton, Sam Rovit and Till Vestring emphasise the importance of having an investment thesis and of getting the planning right from long before the merger announcement—so that the new team can hit the ground running from the first day of the new company's existence.