[en] The concept of squeezing is introduced within the context of quantum optics. It is shown that in a squeezed state one quadrature component of the electromagnetic field exhibits fluctuations that are smaller than in the vacuum, at the cost of a corresponding increase in the fluctuations of the other quadrature. This makes squeezing sensitively dependent on the phase of the field, and accounts for some of the technical difficulties involved in observing and using the effect. In order to measure the desired quadrature of the field one can perform a homodyne or heterodyne experiment with a coherent reference beam, and study the resulting fluctuations with the help of a photoelectric detector. The presence of a squeezed state is then characterized by the observation of sub-Poissonian photoelectric counts, or photoelectric fluctuations below the shot noise limit. Finally, the concept of higher order squeezing is discussed