A billion people now have iPhones. According to my estimate, Apple surpassed the billion iPhone users milestone last month. Thirteen years after going on sale, the iPhone remains the perennial most popular and best-selling smartphone. Competitors continue to either shamelessly copy iPhone or, at a minimum, be heavily influenced by the iPhone. Looking ahead, Apple’s top priorities for the iPhone include finding ways to keep the device at the center of people’s lives while at the same time recognizing the paradigm shift ushered in by wearables.