The term acoustic emission (AE) is widely used to denote the phenomenon in which a material or structure emits elastic waves of shock type and sometimes of continuous type caused by the sudden occurrence of fractures or frictional sliding along discontinuous surfaces. This phenomenon occurs in a variety of cases, and so it has been studied in various fields, such as material science, civil engineering, mining engineering, seismology. Therefore, at the early stages the phenomenon was studied independently in different fields and different terms were used in different fields, that is, for example, acoustic emission in metal engineering and microseismic activity in mining engineering. The present author used the term elastic shock in his early papers. Since the 1970s, the term acoustic emission has been widely used. In this chapter, some subjects of laboratory studies on acoustic emission phenomena in rocks and rock-like materials will be mentioned mainly on the basis of the author’s experiments.