Mobile phone use is booming. There are close to 3.5 billion mobile phones in use, and mobile penetration rates are increasing quickly, especially in developing countries. This rise of mobile phone use by low-income and so-called 'base-of-the-pyramid' users raises a number of questions. Are low-income people using mobile technology in different ways than their higher-income counterparts? How can mobile phones be desiged and used in ways that are useful to these populations? Two new studies--one of favelas in Brazil and the other of a low-income township in South Africa--seek to answer these questions.