In the last several years there have been studies and experiments showing how, by applying an additional laser beam, optically-thick transitions may be rendered nearly transparent to probing radiation. This transparency results from a quantum interference, very much like a Fano interference, which is established by the additional laser. This talk describes the difference between the quantum interference as exhibited by an independent atom and by an optically-thick ensemble of atoms. We find that an ensemble of atoms establishes transparency through a strong nonlinear interaction which, for a lambda system, tends to generate a matching temporal envelope on the complementary transition. For a ladder system, phase conjugate pulses are generated and, after a characteristic distance, establish transparency. The transparency of an optically-thick medium is therefore not a Beer`s law superposition of the independent atom response. To transmit a pulse through an otherwise opaque media, the front edge of the complementary pulse should lead, in the manner of {open_quotes}counter-intuitive{close_quotes} adiabatic transfer, the front edge of the pulse which is to be rendered transparent. Thereafter the pulses should be matched or, for a ladder system, phase-conjugately matched.