This is an excerpt of a white paper I wrote for The Application Developers Alliance.

You might think that in a perfect world, application developers would write code just once and it would run everywhere. But that mobile utopia doesn’t exist. Instead, developers practice their craft in a digital universe that includes multiple versions of competing operating systems and thousands of devices in various screen sizes.

This fragmentation forces developers and the companies that hire them to answer tough questions every time they imagine the next big thing: Should they build for one operating system over another? Should they develop only for the big two, Android and iOS? If so, should they build the apps simultaneously, and should they design one app for both or two different apps for the two platforms? Should they use tools designed to streamline cross-platform development? How should they test the app for the hundreds of Android options? Or should they go to the Web via HTML5?

All of those questions and more are behind the mobile industry’s pursuit of interoperability, the term that describes the ability of apps to work on numerous devices and platforms. “Right now it’s frustrating,” said Peter Braxton, the founder and CEO of Jump Rope Inc. “For someone who’s trying to build a business and presence on different platforms, it’s really hard because it’s additional investment in resources, both time and capital, each time you encounter a new platform.”

Pure interoperability is not technically possible. It also doesn’t consider the cultural, regional and language differences of customers, or their preferred user experiences or devices. Every market has its own peculiarities (try asking a Finnish person to fill in the “State” field in an address form); every device has its own style. In short, there are many reasons to have different versions of an app for different devices, markets and groups of users.

That being said, app makers have sound business reasons to maximize the available market for an app without technical constraints. This white paper defines the challenge, explains why it matters and discusses the options available to developers, including tools to help both developers and non-developers build interoperable products more quickly. The paper concludes with case studies based on the efforts of several experts in the app industry to achieve interoperability or help others do so.

Read the full paper and case studies.