This paper reviews the design and implementation of hardware and software tools for nonintrusive electrical load monitoring. Estimates of spectral content in measured waveforms can be used to determine in real time the operating schedule of loads at a target site. Techniques for transient event detection are reviewed. These techniques can detect the turn-on and turn-off transients of individual loads, and can be used to easily determine the energy usage of loads that draw constant power in steady-state operation. Techniques for monitoring the power consumption of smoothly varying lends (e.g., variable speed drives) using spectral estimates are also discussed.