The ATLAS Forward Calorimeters (FCal) cover the very forward region of 3.1 < |η| < 4.9 and are hence designed to operate in a very high rate environment. This paper discusses the design of the FCal and presents studies of its performance in beam test data and LHC collision data. Measurements of the signal pulse shape, and the distribution of reconstructed cell energies are presented and compared with the expectation from simulations. A measurement of the location of the FCal is also performed in the data by studying the average deposited energy as a function of transverse distance to the beam axis.