The market cap (market capitalization) is the number of shares out x the share price. That's it. If shares are bought back, the share price may or may not change. There would be less dilution, yes, but that has nothing to do with market cap. It is probably the simplest equation in the stock market. OS X SP = Market cap. So in principle, the market cap would DROP with a buy back, although we all know that the share price would increase, but that is due to demand increasing.