A merger of two companies, if the merger gains regulatory approval, means shareholders of both companies gain a roughly equal stake in the new company. This is unlike an acquisition, where one side either swallows the newcomer or parks it under a holding company.
Mergers are generally handled by investment banks that earn significant fees from such deals through buying and selling the two companies' shares. This leads some critics to accuse such banks of conflicts of interest in pumping up such deals and creating inflated expectations for the deal.
The Human Factor
Although around 70 percent of all mergers (and combined M&As) worldwide fail to live up to promise, companies continue to seek them for several reasons, ranging from joining a competitor to boost market share to acquiring a loss-maker as a tax write-off.
Misplaced enthusiasm for the deal on both sides has sunk recent high-profile corporate mergers like AOL with Time Warner and Citigroup with Travelers, underlining the difficulties in getting mergers to work. Most flops are due to human foibles like failure to plan the transaction thoroughly, failure to think the merger through or a lack of objectivity, Wharton Business School reports, and losing good employees in the process is so common it is referred to as "merger syndrome".
Meanwhile, rivals at Harvard Business School (HBS) have compiled a "nine deadly sins" list in typical merger failures and stressed the need establish an 'integration team' early on to avoid them. The HBS paper also points out that 70 percent of the corporate mergers that fail do so at the execution stage of the deal, not during its original planning. The key, HBS concludes, is "selecting the right person or people to lead the program integration team and track the plan's execution."
- On December 20, 2007, Eurex and ISE announced that they had completed a merger. At the time, the collaboration was called the largest transatlantic derivatives marketplace. 
- On April 4, 2007, the New York Stock Exchange announced a merger with Euronext, which brought together six cash equities exchanges in five countries and six derivatives exchanges. 
- In July 2007, the CME Group and the CBOT agreed to merge after nine months of back and forth as well as a surprise bid from ICE. The crosstown rivals merged and dominate the US futures market. 
- U.S. regulator the Federal Communications Commission (FCC) in late July 2008 approved a merger between America's only two satellite-radio distributors, Sirius and XM, creating what some industry critics call an unfair monopoly. Shares of both companies, which had been battling the FCC for more than a year to merge, rose after the decision was announced but soon returned to their normal trading range.
- On February 9, 2011, NYSE Euronext and Deutsche Boerse confirmed that they were in advanced talks on a possible merger.  Also announced on the same day, advanced merger talks between the London Stock Exchange and the Toronto Stock Exchange. 
- What is a Merger?. WiseGeek.com.
- Why Do So Many Mergers Fail?. Wharton Business School.
- Nine Steps to Prevent Merger Failure. Harvard Business School.
- Eurex and ISE complete merger. ISE website.
- NYSE announces merger with Euronext. NYSE website.
- CBOT Holders Choose to Merge with CME. Forbes.
- FCC approval allows for merger of XM, Sirius subscription satellite radio services—with stipulations. MerersUnleashed.com.
- NYSE Euronext and Deutsche Boerse in Advanced Talks to Merge. Bloomberg.
- LSE to buy Canada's TMX. Bloomberg.