Many of the applications of photonics to broadband diagnostics experiments require modulating light in response to an electrical input signal. The two general methods which are employed for this purpose are direct modulation of a source and modulation of a source by an external device. Direct modulation of a source (light emitting diode, laser diode, etc.) involves the relatively straightforward application of well known electrical tech niques and, depending on the required bandwidth, linearity, and dynamic range, may be implemented with a few components at low cost. These techniques are treated in detail in other chapters.