The observation of extragalactic cosmic rays requires detectors with an extremely large surface area, due to the very limited flux. As a result, extragalactic cosmic rays are generally detected with ground-based observatories, by means of the extensive air showers they create. These ground based observatories can be either surface detectors, which observe the air shower particles which reach the ground, or air fluorescence detectors (also called 'fly's eye' detectors), which observe the fluorescence caused by the interaction of the charged air shower particles with the atmosphere. In either case, the ultimate aim is to find the mass and energy of the primary cosmic ray which created the shower. Surface detectors accomplish this by measuring the density of particles at the ground, while fluorescence detectors do so by measuring the depth of shower maximum (the depth from the top of the atmosphere at which the maximum number of particles are present in the shower). The two currently operating high energy cosmic ray observatories, the Pierre Auger Observatory and the Telescope Array, are hybrid detectors which use both of these methods. This hybrid methodology allows for a full three-dimensional reconstruction of the air shower, and gives much better directional information as well as more accurate determination of the type and energy of the primary cosmic ray than either technique on its own.