  The aim of the Pierre Auger Observatory is the investigation of the nature of
cosmic ray particles at ultra-high energies. It can simultaneously observe the
longitudinal air shower development in the atmosphere as well as particle
densities on the ground. While there are no dedicated muon detectors,
techniques have been developed to estimate the number of muons, $N_\mu$,
produced by air showers. Both, the longitudinal development, in particular the
depth of the shower maximum, $X_{\rm max}$, and the muon content of air showers
are highly sensitive to hadronic interactions at ultra-high energies.
Currently, none of the available hadronic interaction models used for
simulations of extensive air showers is able to consistently describe the
observations of $X_{\rm max}$ and $N_\mu$ made by the Pierre Auger Observatory.
