Melvyn Bragg and his guests discuss Euler's number, also known as e. First discovered in the seventeenth century by the Swiss mathematician Jacob Bernoulli when he was studying compound interest, e is now recognised as one of the most important and interesting numbers in mathematics. Roughly equal to 2.718, e is useful in studying many everyday situations, from personal savings to epidemics. It also features in Euler's Identity, sometimes described as the most beautiful equation ever written. With: Colva Roney-Dougal Reader in Pure Mathematics at the University of St Andrews June Barrow-Green Senior Lecturer in the History of Maths at the Open University Vicky Neale Whitehead Lecturer at the Mathematical Institute and Balliol College at the University of Oxford Producer: Thomas Morris.