The reciprocity technique is the most highly developed and most widely used primary calibration method at the major national standards laboratories of the world. Critical measurements of sound pressure in air are usually traceable to such calibrations, which are briefly described with reference to specific apparatus and procedures at the NBS. Secondary methods, including those based upon reciprocity, can offer improved convenience with minor to significant sacrifices in accuracy and precision. Dynamic signal analyzers incorporating digital signal processing are now sufficiently accurate and precise to offer promise for improved and/or more readily automated measurements, especially secondary free-field calibrations. Specific examples from the NBS research and measurement services are discussed.