Amplitude and frequency characteristics of acoustic emission signals are estimated by the initial and boundary conditions of the dynamic equilibrium equation for a linear elastic medium. Except in some special cases, frequency responses of signals is controlled only by the frequency response function of specimens, in addition to the characteristic of the transducer. In these cases, the source mechanism of acoustic emission can be attributed to the above initial condition. This initial condition is decided by inhomogeneous deformation with respect to time and space, which gives a local inelastic strain Δε during the time interval ΔT(relaxation time) in the material. From a metallurgical viewpoint, it can be recognized that this local inelastic strain is introduced by stress concentration τbaround various inhomogeneities, such as grain boundary, cell boundary, inclusion, and precipitate, and relaxation time ΔTis controlled by frictional stress τfin the matrix. According to this model, acoustic emission signals can be well understood from a viewpoint of inhomogeneous deformation, which is described by stress terms τband τf. In addition to a historical survey of source mechanisms of acoustic emission, in this report emission signals during various type of yielding are analyzed and discussed from a viewpoint of inhomogeneous deformation.