An introduction to the effects of light on semiconductor devices and their use in testing integrated circuits is given. The possibilities are discussed of utilizing a focused laser beam as a minute, movable, non-destructive probe. This may be used to feed information into integrated circuits, to locate and simulate faults, and to shorten the time required to test these circuits. Experiments performed have shown that a few microwatts of laser beam power can switch internal nodes in m.o.s. digital circuits in less than a microsecond. By using an appropriately modulated beam binary sequences have been injected in a similar manner. The beam was also used to determine the minority carrier diffusion length in m.o.s. digital circuits.