2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/171311It is a fundamental consequence of the superposition principle for quantum states that there must exist non-orthogonal states, that is states that, although different, have a non-zero overlap. This finite overlap means that there is no way of determining with certainty in which of two such states a given physical system has been prepared. We review the various strategies that have been devised to discriminate optimally between non-orthogonal states and some of the optical experiments that have been performed to realise these.43 pages, submitted to Advances in Optics and PhotonicsQuantum Physicstext