GCHQ is a top-secret intelligence and security agency which recruits some of the very brightest minds. Over the years, their codebreakers have helped keep Britain safe, from the Bletchley Park breakthroughs of WWII to the modern-day threat of cyberattack. So it comes as no surprise that, even in their time off, the staff at GCHQ love a good puzzle. Whether they're recruiting new staff or challenging each other to the toughest Christmas quizzes and treasure hunts imaginable, puzzles are at the heart of what GCHQ does. Here in this book they open up their archives to share decades' worth of codes, puzzles and challenges for everyone to try.