Forty wealthy families and individuals join Microsoft Corporation co-founder Bill Gates and billionaire investor Warren Buffett in a pledge to give at least half their wealth to charity.

Six weeks after launching a campaign to get other billionaires to donate most of their fortunes, the chairman and CEO of Berkshire Hathaway Incorporated released the first list of people who have signed what he and Gates are calling the "giving pledge."

Buffett decided in 2006 to give 99 percent of his roughly $46 billion fortune to charity.

In addition to making a donation commitment, Gates and Buffett are asking billionaires to pledge to give wisely and learn from their peers.