Click a few keys, exchange a few numbers, and it's done. With just a mobile phone and a registration with Safaricom, Kenya's mobile service giant, you can pay for anything in seconds - no cash, no long journeys to towns to reach a bank, and no long lines when you get there. This is m-Pesa, the revolutionary approach to banking which is changing economies across Africa. The service allows customers and businesses to pay for anything without needing cash, a bank account, or even a permanent address. In today's Digital Diversity, in honour of its recent fifth birthday, we present a beginner's guide to m-Pesa and examine its implications for financial access in developing economies.