Quantum theory confronts us with bizarre paradoxes which upset the logical edifice of classical physics. At the sub-atomic level, one particle seems to know what the others are doing (so-called 'non-locality'), and according to Heisenberg's 'uncertainty principle', there is a limit on how accurately nature can be observed. And yet the theory is amazingly accurate and widely applied, explaining all of chemistry and most of physics. This book takes us on a step-by-step tour with the key figures, including Planck, Einstein, Bohr, Heisenberg and Schrodinger. Each contributed at least one crucial concept to the theory. The puzzle of the wave-particle duality is here, along with descriptions of the two questions raised gainst Bohr's 'Copenhagen Interpretation' - the famous 'dead and alive cat' and the EPR paradox. Both remain unresolved at the beginning of the 21st century.