Abstract : To make an atom transparent at a given laser frequency, one applies a second laser whose frequency is equal to the difference of an otherwise empty state and the point in frequency space to which a probing laser is tuned. This type of transparency exhibits an essential nonreciprocity where, though absorption and refractive index may be negated, the nonlinear susceptibilities and coefficients for stimulated and spontaneous remain unchanged. We believe that there may be a new regime of nonlinear optics with special properties as resonances are approached.... Electromagnetically Induced Transparency.