One of the most exciting recent changes in the computing world is cloud computing. Cloud computing is a dramatic shift in how applications are developed and used---and even in what applications are. With cloud computing, developers are no longer building applications that run on a user's desktop computer. Instead, they're building services on the network that can be used by thousands of users at the same time. Cloud services are an exciting opportunity for developers: the cloud is a platform for creating services, a new kind of application that can reach more users, and provide those users with more capabilities than a desktop application ever could. Building applications as cloud services also makes them scalable: cloud applications can easily and smoothly adapt from running on a single computer for a single user to running on thousands of computers for millions of users. Code in the Cloud will teach you what a cloud service is, and how it differs from traditional applications. It will show you how to build a cloud service, taking advantage of the services that AppEngine makes available to you, using iterative development of a simple application to guide you through the different aspects of AppEngine development, using either Python or Java. Through the process of working on a simple application, you'll learn about how to build an application as a service; how to manage persistent data using AppEngine; how to build dynamic, interactive user interfaces that run in a user's web-browser; how to manage security in a web application; and how to interact with other services running in the AppEngine cloud.