The “Hadoh” group of societies for the study of thermoacoustics and thermoacoustical engineering, established in 1988, was adjourned in 1977. During the 9 years, members of these societies promoted great progress in thermoacoustics and thermoacoustical engineering, especially in the area of pulse-tube refrigeration. The thermoacoustical engineering supported by the Cryogenic Society of Japan is presently spreading into the Japanese Society of Mechanical Engineering and other societies. A wide variety of thermoacoustic phenomena has been studied, and the thermoacoustic theory has developed from “standing wave approximation” to a theory including “progressive waves” in order to understand thermoacoustic devices equipped with regenerators. While thermoacoustic theory succeeded in a qualitative understanding of thermoacoustic phenomena and has been used for the R&D of thermoacoustic refrigerators, there remain important problems; how to control the steady mass flow and how to understand and design heat exchangers of thermoacoustic devices. These are essential and urgent problems in developing practical devices of higher efficiency. A deep insight of thermoacoustics may be one of the keys to the future development of thermodymnamics that consider time.