Reactive programming is an emerging development methodology, where changes to data automatically trigger changes to the rest of the system. This allows you, the developer, to write code declaratively and let the reactive elements manage any changes. Meteor is, perhaps, the best and most fully developed implementation of reactive programming available today. By understanding the core concepts of reactive programming, you can use the Tracker (formerly Deps) library to create simple, declarative code while avoiding the usual pitfalls associated with reactive and asynchronous JavaScript programming. The recipes in this chapter will give you simple, clear examples of how the major components of Meteor's reactive model work.