This document provides an overview of BeepBeep 3, an open source event stream query engine developed for processing and analyzing event streams. It describes some key concepts including events, traces, functions, processors, synchronous processing, and various built-in processors like Filter, Window, and Cumulative. Processors can be composed together to perform multi-step queries on event streams. The engine aims to combine strengths from rule-based and complex event processing systems while providing extensibility through additional processor and function palettes.