By 2030, two billion people who don't have a bank account today will be storing money and making payment with their mobile phones, predicts Bill Gates.
In their annual letter, Gates and his wife Melinda place mobile banking as one of their five "big bets" that will help transform the lives of the poor by 2030.
The poor have traditionally used financial services that are extremely inefficient, such as cash and commodities that lose value, says the letter. But the mobile revolution will give these people more control over their assets.
This is partly because while it has not been economically viable for banks to serve the poor, the ubiquity of mobile phones and the fact that the marginal cost of processing digital transactions is near zero is changing the game.
"By making small commissions on millions and millions of transactions, mobile money providers can make a profit serving poor customers, just as brick-and-mortar banks do serving the wealthy," say the Gates'.
And as basic services get going, there will be competitive innovation that will see greater access to savings, insurance and credit products.
The letter warns that to ensure that mobile banking succeeds work still needs to be done to improve access for women, to craft regulation that encourages innovation and to have networks of physical outlets for converting digital money into cash.
"But in the next 15 years, digital banking will give the poor more control over their assets and help them transform their lives."