The studies by academics, consulting firms, and the business press all agree—mergers are just as likely to destroy as to create shareholder value. Despite this track record, merger activity looks poised for a rebound. A single day in October 2003 witnessed four deals valued at more than $70 billion, the largest wave of activity since the 1990s bull market.

How can companies entering a merger—especially those planning the largest, most complex mergers—improve their likelihood of success? The list of merger best practices is familiar. Concentrate on synergies. Integrate quickly. Maintain a focus on customers and revenue growth. Communicate continuously. Address human and cultural issues.

Despite these well known prescriptions, roughly half of mergers fail to create value, demonstrating how difficult it is to execute these tasks. How have successful mergers worked? And what lessons can be learned by other firms that are contemplating acquisitions?

To gain a first-person perspective on what makes for merger success, we interviewed senior executives responsible for integration in three recent mega-mergers: Hewlett-Packard with Compaq, Northrop Grumman with Litton Industries, and AmeriSource Health Corporation with Bergen Brunswig Corporation.