High-speed detectors are general-purpose laboratory devices that are useful in a wide variety of high-bandwidth applications. They may be used to provide optical input to high-speed oscilloscopes, spectrum analyzers, network analyzers, or as integral components in experiments where timing, microwave detection, or other needs are met. With recent advances in high-speed communications systems and short-pulse lasers, many applications are pushing the requirements of high-time-resolution/high-bandwidth detectors to faster and faster speeds. For example, telecommunications laboratory systems can reach 40 Gb/s and 100 Gb/s data rates, requiring detectors with bandwidths over 50 or 100 GHz. In addition, in chemical, physical, and biological experiments, picosecond and femtosecond ultrafast laser systems are becoming increasingly common as diagnostic tools for studying fundamental physical processes. High-speed photodetectors can increase resolution in spectroscopy. Moreover, they can be used in laser heterodyning and other experiments aimed at the optical generation and distribution of microwave or millimeter-wave signals. Their use in all of these applications require knowledge of detector characteristics and microwave circuitry, and an awareness of how a detectoris frequency/impulse response influences measurements. In addition, it is important to know whether the application requires measurements in the time or frequency domain, and to choose the detector accordingly.