Melinda convinced Bill to start the BMGF foundation about 23 years ago. They put up what I think was half of their net worth, or $30,000,000,000 (30 billion dollars). Buffet promised to match every yearly expenditure, so roughly another $30 billion. They intend to spend it all down by the time the last trustees dies (probably Melinda). That is like the Rockefeller foundation (still around because compound interest) spent all of the money on charity before he died.