This book marks the centenary of the discovery of the electron by J. J. Thomson in 1897, an event which occurred at a great turning point in the history of scientific ideas, and the impact of which on the development of science in the twentieth century has been profound. The electron was the first elementary particle to be discovered. It sets the basic scales of energy and length in chemistry and materials science, and its ubiquitous presence to drive electrical and electronic devices in everyday life is familiar to everyone. In this book the discussion moves from the historical context of the discovery of the electron and its basic properties, to the Dirac equation, bonding in condensed matter, Fermi and non-Fermi liquids, quantum order, superconductivity, heavy, coherent and composite electrons, and the role of the electron in the cosmos, with each chapter written by a leading figure in the field.