This historical survey of the discovery of the electron has been published to coincide with the centenary of the discovery. The text maps the life and achievements of J.J. Thomson, with particular focus on his ideas and experiments leading to the discovery. It describes Thomson's early years and education. It then considers his career at Cambridge, first as a fellow of Trinity, later as the head of the Cavendish Laboratory and finally as Master of Trinity and national spokesman for science. The core of the book is concerned with the work undertaken at the Cavendish, culminating in the discovery of "corpuscles," later named "electrons."; In the final two chapters, the immediate aftermath and implications of the work are described. These include the creation of the subject of atomic physics as well as the broader long term developments which can be traced from vacuum valves and the transistor through to the microelectronics revolution.