It starts with understanding how the world's poor behave and making products they need—not the products banks want to make. If everyone had a bank account, the global economy could expand by $3.7 trillion by 2025, a recent report showed. New digital tools, like mobile money accounts and peer-to-peer lending, can bring millions of people into the formal financial system, opening up their ability to save, borrow, and live better lives. Already, 700 million individuals gained bank accounts between 2011 and 2014.