  We analyze the effects of Scalar and Vector Leptoquarks on various
observables in electron (positron) - proton deep inelastic scattering. In view
of the future program of the HERA collider, with a high luminosity and also
with polarization, we present the constraints that can be reached using this
facility for several Leptoquark scenarios. We address the question of the
identification of the nature of a discovered Leptoquark. We emphasize the
relevance of having polarized lepton and proton beams in order to disentangle
completely the various Leptoquark models. This study is also relevant in the
context of the TESLA$\times$HERA project.
