An introduction to the key principles of quantum mechanics, beginning with an examination of the quantum-mechanical description of the behaviour of electrons around atomic nuclei. This is followed by an overview of some of the other major principles of quantum theory, including the Heisenberg Uncertainty Principle, the Pauli Exclusion Principle, quantum tunnelling and entanglement. The episode concludes with a discussion of alternative interpretations of these quantum phenomena. Recommended pre-listening is Episode 8: The Atom.