Applications with timeliness constraints are called “real-time”. When an event happens, the application should deal with it within a known and bounded delay. The application taking more time can have a potentially (very) negative impact. Real-time applications are classified in two categories. Hard real-time applications are such that the bounded delay absolutely needs to be met. Examples include forest fire detection. Soft real-time applications are looser in that they can tolerate not to meet a deadline. Examples include multimedia applications. Applications can be classified in four categories, according to how data exchange is initiated. In event-driven application, communication starts when some event happens. In query-driven applications, communication only starts when a specific node sends a query. This specific node can be the sink node in the case of WSNs. In time-triggered applications, communication happens at predefined instants. Finally, some applications such as live video transmission require data to continuously flow through the network. Each one of these application categories has specific time-related constraints.