  The current status and prospects of deducing the proton-air cross section
from fluorescence telescope measurements of extensive air showers are
discussed. As it is not possible to observe the point of first interaction, $X_
{\rm 1}$, directly, other observables closely linked to $X_{\rm 1}$ must be
inferred from the measured longitudinal profiles. This introduces a dependence
on the models used to describe the shower development. Systematic uncertainties
arising from this model dependence, from the reconstruction method itself and
from a possible non-proton contamination of the selected shower sample are
discussed.
