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Google's Android operating system and Microsoft's Windows Phone 7 are the two main challengers to Apple's iPhone platform. These rivals offer two different approaches as they attempt to copy Apple's success in the fast-growing mobile device marketplace.

Visual tour: Windows Phone 7

Google's interest in mobility is driven by its conviction that mobile devices will become the most common and most frequently used means of accessing the Web, the home of Google's powerful search engine and the source of the company's profitability.

The Google-led Open Handset Alliance – now comprising around 70 software, silicon and handset vendors and some carriers –released the open source Android smartphone software platform in November 2007. The first Android smartphones arrived a year later.

The gamble that handset makers and carriers would embrace a free, open source mobile OS and aggressively innovate with it has paid off. Less than three years after Android's advent, smartphones running the new OS outsold iPhones in the United States during the first quarter of 2010, according to data from NPD Group. Globally, Android's market share soared from 1.6% in the first quarter of 2009 to 9.6% in the first quarter of 2010, according to analyst firm Gartner.

Motorola, for example, was on the verge of selling off its failing mobile phone division. Now that group is re-inventing itself, based on Android. Motorola crafted its own user interface over the Android OS, and the company is considering using Android in new classes of mobile devices, such as tablets. The well-received Motorola Droid smartphone, unveiled in October 2009, was popular and well-reviewed, winning favorable comparison to the iPhone.

For Microsoft, the stakes are equally high. Its dated Windows Mobile UI, built on top of the Windows CE kernel, was losing marketshare and its latest iteration, Windows Mobile 6.5, underwhelmed the industry in 2009. By then, CEO Steve Ballmer had reorganized the mobile effort, with new leadership in engineering and marketing. The re-engineered user interface, now dubbed Windows Phone, was unveiled in February 2010. In May 2010, another Ballmer shakeup separated the Xbox gaming and Windows Phone units, with both division chiefs now reporting directly to Ballmer, rather than through Robbie Bach.

The look and feel of the new UI is radically different, not only from Microsoft's previous efforts but from rival platforms. Microsoft hired talented outsiders, paired them with Redmond veterans, and gave them a blank sheet and nearly a blank check. One measure of Microsoft's seriousness is that Windows Phone scraps its long-standing principal of backward compatibility: applications written for the older Windows Mobile OS have to be rewritten for Windows Phone.

In general, Microsoft application developers have been favorably impressed with the new UI, and they've praised the quality of the early code and of the early versions of the developer tools. Windows Phone 7 has been favorably compared with the iPhone OS.

Microsoft plans to continue its business model of platform licensing, but it's forged closer ties with handset vendors and chipmakers, creating a joint specification to ensure a high-quality mobile experience for end users. Two challenges for Microsoft remain: to create a viable online software marketplace and to demonstrate that Windows Phones can tie in seamlessly with PC- and cloud-based data and applications.

In mid-2010, Android clearly has momentum. By the fall of 2010, Microsoft will have to show a pack of exciting handsets to reclaim the hearts and minds of consumer and business users alike.