Book excerpt: The simple text message application that appears on virtually all mobile phones is the ultimate thin client, allowing your users access to the full computing power and informational depth of the Internet from a cheap cell phone on a mountaintop. Building an SMS service can be quite simple. This tutorial guides you through a variety of implementations, giving you the information you need to choose one that best fits your unique needs and circumstances. More than that, though, it seeks to help you understand the core principles necessary to make your service a success.