As fiber and free-space optical communication bandwidths increase, the need for very high speed optical modulators and detectors has also increased. The frequency response characterization of these electrical-to-optical (E/O, modulators sometimes integrated with lasers) and optical-to-electrical (O/E, detectors and receivers) converters can be important in terms of such parameters as bandwidth, flatness and phase linearity. In addition, the microwave frequency response characteristics of certain purely optical (O/O) components may be of interest as their optical bandwidth may be narrow (couplers, amplifiers, filters, etc.) or may have unusual dispersion characteristics. The MS464XX VNA has a number of measurement utilities to facilitate this kind of analysis and, coupled with the MN4765X O/E calibration module or some other calibration device, some level of measurement traceability is possible. The ME7848A series of systems incorporate the MS464XX VNA, MN4765X O/E module and, optionally, a MN4775X laser/modulator unit. This chapter will discuss some of the measurements of interest, setup considerations, possible measurement performance, and examples of execution procedures.