Overwhelming numbers are scary—even in the best of circumstances. You can plan for them, build up your defenses, and do everything imaginable to prepare. But when that horde of zombies hits, their sheer numbers can still cause devastation. Botnets are digital zombie hordes. Jamie Tomasello recounts the scale of the Bredolab botnet—and the many malicious kinds of missions it carried out. Martijn Grooten explains how botnets work, and why they can be so difficult to permanently dismantle. And Darren Mott shares some of the successes the FBI had in rounding up some of the world’s most prolific bot herders.