  Studies of the influence of different hadronic models on extensive air
showers at ultra-high energies are presented. The hadronic models considered
are those implemented in the well-known QGSJET and SIBYLL event generators. The
different approaches used in both codes to model the underlying physics is
analyzed using computer simulations performed with the program AIRES. The most
relevant observables for both single collisions and air showers are studied for
primary energies ranging from $10^{14}$ eV up to $10^{20.5}$ eV. In addition,
the evolution of lateral and energy distributions during the shower development
is presented. Our analysis seems to indicate that the behaviour of shower
observables does not largely reflect the strong differences observed in single
collisions.
