Book excerpt: A fundamental step towards gaining a deeper understanding of our world is to increase the resolution of the investigative instruments we use; i.e. to increase the energy, and hence to decrease the wavelength, of the particles which constitute our probes. Almost any substantial progress in our understanding of the fundamental laws of Nature has been obtained when a new generation of accelerators has allowed us to achieve a new energy range. The new results have generated new questions, thus encouraging us to construct new machines to reach even higher energy levels. The relative energy gain from one generation of accelerators to the next is progressively increasing. The energy ga in suggested by the theoretical predictions at the time has usually been much greater than the value allowed by our technical capabilities. But this smaller energy gain permitted by accelerator technology improvement has generally been sufficient up until now to bring about a substantial increase in our knowledge. Hence a large increase in accelerator energy is very important, and we know that this result can essentially be obtained by developing some new device or some new approach.