  The Recycler Electron cooler was the first (and so far, the only) cooler
working at a relativistic energy ({\gamma} = 9.5). It was successfully
developed in 1995-2004 and was in operation at Fermilab in 2005-2011, providing
cooling of antiprotons in the Recycler ring. This paper describes the cooler,
difficulties in achieving the required electron beam parameters and the ways to
overcome them, cooling measurements, and details of operation.
