Smartphones and tablets are game changers for engagement because people carry them everywhere they go. Your customers and partners and employees have perpetual access to the vast resources of the Internet — and hopefully your mobile app as well — in the key moments of their day. In interviews with more than 100 mobile innovators, we have found four places where mobile engagement transforms services and processes: customer engagement, process acceleration, employee productivity, and new business services. Even at this early stage of development, you can see positive impacts of mobile engagement in apps from General Electric, Trane, and Uber Technologies. We close with a simple framework for prioritizing which apps and features people will actually use based how useful and convenient they are.