  The Pierre Auger Observatory is a facility designed for the study of
ultra-high energy cosmic rays. The Observatory combines two different types of
detectors: a surface array of 1600 water Cherenkov stations placed on a 1.5 km
triangular grid covering over 3000 km$^2$; and a fluorescence detector of 24
telescopes located in 4 buildings at the perimeter of the surface array. The
fluorescence telescopes, each consisting of 440 photomultipliers, collect the
ultraviolet light produced when the charged secondary particles in an air
shower excite nitrogen molecules in the atmosphere. Because the intensity of
the nitrogen fluorescence is proportional to the energy deposited in the
atmosphere during the air shower, the air fluorescence measurements can be used
to make a calorimetric measurement of the cosmic ray primary energy. Showers
observed independently by the surface array and fluorescence telescopes, called
hybrid events, are critical to the function of the Observatory, as they allow
for a model-independent calibration of the surface detector. In this paper I
describe the detector and the most important measurements.
