If 3=T, 4=S, 5=P, 6=H, 7=H ...what is 8? What is the next letter in the sequence: M, V, E, M, J, S, U, ? Which of the following words is the odd one out: CHAT, COMMENT, ELF, MANGER, PAIN, POUR? 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 our country 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. Now they're opening up their archives of decades' worth of codes, puzzles and challenges for everyone to try. In this book you will find: - Tips on how to get into the mindset of a codebreaker - Puzzles ranging in difficulty from easy to brain-bending - A competition section where we search for Britain's smartest puzzler Good luck!