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The determination of the mass composition of the highest energy cosmic raysis one of the greatest challenges in cosmic ray experiments. The highest energycosmic rays are only detected indirectly because of their very low flux. Usingthe atmosphere as a large target, Air Fluorescence Detectors are capable oftracing the evolution of the size of the Extensive Air Shower through theatmosphere (the shower longitudinal profile). The analysis of thecharacteristics of the detected longitudinal profiles is currently the mostreliable way for extracting some information about the primary cosmic ray masscomposition. In this proceeding, I will describe in some detail the PierreAuger elongation rate studies, and I will show the potential for masscomposition studies using the surface and the fluorescence detectorsinformation as part of a single analysis. The interpretation of the currentdata with regard to mass composition, relies heavily on high energy hadroninteraction models. Using standard hadron interaction models, the data suggestthat the composition becomes lighter up to about 2 $\times$ 10$^{18}$ $eV$ andabove that it becomes heavier again. This apparent change in the masscomposition at 2 $\times$ 10$^{18}$ $eV$ seems to be correlated with a spectrumindex change in the observed energy spectrum.

Mass Composition Studies of the Highest Energy Cosmic Rays