The Free Electron Laser at Jefferson Lab: The Technology and the Science

We discuss the physics and technology which is driving the evolution of high-brightness fourth-generation accelerator-based sources. Some of the first user facilities are in the THz and IR spectral ranges. Specifically, a new generation of sources, called energy recovery linacs, will be described and reviewed with particular emphasis on their utility for imaging, spectroscopy, and dynamics, as well as their novel applications to nonlinear phenomena.