This document provides an overview of complex event processing (CEP) and the WSO2 CEP product. CEP involves analyzing streams of event data for patterns that match predefined rules. It is useful when dealing with large volumes of data that are not needed long-term and time-critical analysis is required. The WSO2 CEP product supports distributed, scalable, and real-time complex event processing using the Siddhi query language. It allows defining queries with filters, windows, aggregations, and patterns to detect opportunities, threats, and anomalies. New features in version 4.0 include improved distributed processing, high availability, and support for dashboards, websockets, and MQTT. Common use cases for CEP include