  The role of Horizontal Showers induced by cosmic rays is discussed in detail.
A new approach to the calculation of the muon component in horizontal air
showers induced by protons, heavier nuclei or photons is presented which allows
a simple analytical evaluation of the muon density profiles at ground level.
The results of the first application of these results to horizontal air showers
detected at the Haverah Park Array by the recently started {\sl Leeds-Santiago
collaboration}, leading to important restrictions on composition at ultra high
energies, are reported.
