Nine out of 10 mobile devices run either Android or iOS operating systems. They share that 90 percent more or less equally. A half dozen other OS’s, including Windows Phone, fill out the remaining 10 percent. Given that there are mobile devices in the hands of about 2.5 billion people today, to ignore either Android or iOS for your consumer-level mobile app development is essentially writing off over a billion devices. Nor is it wise to write off Windows Phone’s growing market share.