The paper reviews and discusses the background, methodology and application fields of absolute calibration of acoustic emission transducers by means of the reciprocity method. For characterizing and calibrating acoustic emission transducers, a number of methods utilizing various mechanical sound sources appear in the literature. Reciprocity calibration of acoustic emission transducers was proposed some time ago, and has since been employed by transducer manufacturers and laboratories. An outstanding advantage of the reciprocity method is that absolute sensitivity, including frequency characteristics and impulse responses, both to Rayleigh wave and longitudinal wave, can be determined by means of purely electrical measurements without the use of mechanical sound sources or reference transducers. The procedure for calibration was standardized as NDIS 2109 by the Japanese Society for Non-Destructive Inspection.