Introduction Computers are everywhere. We all need to learn how to use them, and many of us use them every day. But how do they work? How do they think? And how can people write software that is fast and easy to use? Computer science is a fascinating subject that explores these very questions. The easy and fun activities in this book, designed for studentren of all ages, introduce you to some of the building blocks of how computers work—without using a computer at all! This book can be effectively used in enrichment and extension programmes, or even in the regular classroom. You don't have to be a computer expert to enjoy learning these principles with your students. The book contains a range of activities, with background information explained simply. Answers to all problems are provided, and each activity ends with a 'what's it all about?' section that explains the relevance of the activities. Many of the activities are mathematically based, e.g. exploring binary numbers, mapping and graphs, patterns and sorting problems, and cryptography. Others link in well with the technology curriculum, and the knowledge and understanding of how computers work. The studentren are actively involved in communication, problem solving, creativity, and thinking skills in a meaningful context. The activities also provide a very engaging way to explore " computational thinking " , which is gaining traction in school curricula. In addition to this book, the " Unplugged " project has as lot of free, online resources including videos, pictures and extra material at csunplugged.org. As part of the 2015 revision of this book, we have also released a brand new website, with more resources, better access to the open source material, and stronger curriculum links to match the appearance of computer science and computational thinking in school curricula. This book was written by three computer science lecturers and two school teachers, and is based on our experience in classrooms as well as feedback from hundreds of educators over two decades. We have found that many important concepts can be taught without using a computer—in fact, sometimes the computer is just a distraction from learning. Often computer science is taught using programming first, but not every student finds this motivating, and it can be a significant barrier to getting into the really interesting ideas in computer science. So unplug your computer, and get ready to learn what computer science is …