Abstract
This article reviews the status of Free-Electron Lasers (FELs) working in the terahertz frequency range. At this time, three operating FEL facilities exist that are able to supply powerful and tunable terahertz radiation for various application programs. This article describes the performance of FELs in these facilities and also presents novel radiation sources based on synchrotron radiation. Recent progress in accelerator technology, particularly in the energy recovering linac and electron beam compression technology, opens up the possibility of a terahertz radiation source that produces terahertz waves with MW peak power and KW average power.
