This document compares the three main mobile operating systems: Android, Windows, and iOS. It provides an overview of mobile operating system structure and platforms. Android is based on the Linux kernel and was developed by Android Inc. which was later acquired by Google. Windows Mobile is a compact OS designed for mobile devices based on Microsoft Win32. iPhone OS, now called iOS, is based on Mach kernel and runs on iPhones and iPads. It provides preemptive multitasking like Mac OS X. The document concludes that Android is the most widely used mobile operating system.