This is a book about the 'Halting Problem', arguably the most (in)famous computer-related problem: can an algorithm decide in finite time whether an arbitrary computer program eventually stops? The Halting Problem is 'hidden' in many subjects, from logic (is mathematics free of contradictions?), physics (is quantum randomness perfect?), to philosophy (do humans have free will, or do our brains generate our thoughts and decisions in a deterministic way?) and quantum computing (why we don't have a quantum Halting Problem?) — this book will visit each of them.