Most companies now maintain an alliance portfolio comprising multiple simultaneous alliances with different partners. In the global air transportation industry, for example, most airlines maintain broad portfolios of code-sharing alliances with other carriers, which allow them to significantly extend their route networks by offering services to their partners destinations. In 1994, the average number of alliances per airline company was only four. By 2008, however, the picture had changed dramatically: The average alliance portfolio size across the industry had increased to 12, with some airlines engaging simultaneously in as many as 30 or 40 alliances. Despite this proliferation of corporate collaborations, research reveals a troublesome pattern. When a company adds a new alliance to its portfolio, it tends to focus on how much value the alliance will create as a stand-alone transaction but ignore the fact that the composition of its entire alliance portfolio is an important determinant of the value that will come from a new alliance. In other words, an alliance opportunity that promises to create value from a stand-alone perspective may not necessarily be value-creating from an alliance portfolio perspective. The formation of the new alliance may even be an overall value-destroying move. By studying the global air transportation industry, we found concrete evidence of this proposition.