Event processing is a method of tracking and analyzing (processing) streams of information (data) about things that happen (events), and deriving a conclusion from them. Complex event processing, or CEP, consists of a set of concepts and techniques developed in the early 1990s for processing real-time events and extracting information from event streams as they arrive. The goal of complex event processing is to identify meaningful events (such as opportunities or threats) in real-time situations and respond to them as quickly as possible.

These events may be happening across the various layers of an organization as sales leads, orders or customer service calls. Or, they may be news items, text messages, social media posts, stock market feeds, traffic reports, weather reports, or other kinds of data. An event may also be defined as a "change of state," when a measurement exceeds a predefined threshold of time, temperature, or other value.

Analysts have suggested that CEP will give organizations a new way to analyze patterns in real-time and help the business side communicate better with IT and service departments. CEP has since become an enabling technology in many systems that are used to take immediate action in response to incoming streams of events. Applications are now to be found (2018) in many sectors of business including stock market trading systems, mobile devices, internet operations, fraud detection, the transportation industry, and governmental intelligence gathering.

The vast amount of information available about events is sometimes referred to as the event cloud.