This document provides an overview of WSO2 Complex Event Processor (CEP). It discusses key CEP concepts like event streams, queries, and execution plans. It also demonstrates various query patterns for filtering, transforming, enriching, joining, and detecting patterns in event streams. The document outlines the architecture of CEP and shows how to define streams, tables, queries, and adaptors to integrate CEP with external systems. It provides examples of windowing, aggregations, functions, and extensions that can be used in Siddhi queries to process event streams.