Zeek (formerly Bro), is the world's most popular platform for network security monitoring. Flexible, open-source, and powered entirely by defenders. Zeek has a long track record in the open-source and digital security industries. Vern Paxson started the project under the name "Bro" in the 1990s to help him understand the activities at his university and national lab networks. In late 2018, Vern Paxson and the leadership team of the project renamed Bro and Zeek to celebrate its growth and continued development. Zeek is not an active security device like a firewall, intrusion prevention system, or intrusion detection system. Zeek is a "sensor", a hardware, cloud, or software platform that quietly and inconspicuously monitors network traffic. Zeek interprets what it sees, creates compact, high-fidelity transaction records, files content, and customizes the output. This can be used for manual review on disk, or in an analyst-friendly tool such as a security and event management (SIEM), system.