Optoelectronics is a science and a technology based on the conjunction of optics with electronics. The special advantages of such a conjunction should become clear as the reader progresses through the book but, broadly, they result from the large speed and information-carrying capacity of light coming into alliance with the ready controllability of electrons. 8 1 The great speed of light is weil known ( ...... 3 x 10 m s- in a vacuum); it is, in fact, the greatest speed which is possible for any form of physical energy, as was elucidated by Einstein in his special theory of relativity (1905); and it is not so very long ago ( c. 1640) that many scientists held the view that the speed of light was infinite. The large information-carrying capacity of light derives from the high frequency of its oscillations. The implication is that optical processes occur 14 on a time-scale of order one optical period ( ...... 10- sec), and thus that they can absorb and convey information on the same kind of time-scale, leading to information rates of the order of the optical frequency. Electrons, unlike photons, carry electrical charge. As a consequence they respond to electric and magnetic fields, which thus can be used to control and manipulate them. This is a significant advantage, since mod ern electronics provides finely adjustable fields at a sophisticated Ievel of flexibility.