This paper reviews the self-mixing interference (SMI) in terms of its operation principle, the features of SMI signals and its configuration. SMI refers to a phenomenon that occurs when a small fraction of the light emitted by a laser is backscattered or reflected by an external target and re-enters the laser active cavity, thus leading to the modulation of the laser output power. This is a remarkably universal phenomenon, occurring in lasers regardless of type. A few application examples are presented based on the research work done in our group, including SMI sensing for displacement measurement, material parameters and laser parameters. An SMI with the laser operating at the relaxation oscillation is introduced which has potential for achieving more sensitive sensing.