Test beam results from the CMS Zero Degree Calorimeters

The CMS Zero Degree Calorimeters are designed to measure forward neutrons and photons in TeV scale pp and heavy ion collisions. We will present test beam results from electron and pion beams in the energy range of 20-350 GeV. We will discuss the resolution and linearity of the detector as a function of energy. For pp collisions we expect to see protons with energies as high as the beam energy, 7TeV. This energy is 20 times larger than the highest available test beam energy and so calibration is a particular challange. Finally we will describe how we plan to use the detector for early physics measurements at the LHC.