This document summarizes the state-of-the-art in event processing tools. It discusses the history and evolution of complex event processing systems and data stream management systems. The document then characterizes over 20 modern distributed event processing tools based on their release year, capabilities, and relationship to Hadoop. It concludes that while these tools have improved scalability using techniques like MapReduce, their convergence and specialization is still uncertain, and more evaluation is needed to understand performance and applicability to high-performance computing.