This is a tutorial review on the subject of mode selection in lasers. We begin with a historical review. After an introduction to the subject of modes in laser resonators and a brief review of the theory of laser gain saturation, the main body of the paper is devoted to a discussion of various mode-selection techniques, many which can be used to produce single-frequency laser operation. We discuss some systems for frequency stabilization of single-frequency lasers, and conclude with examples of laser applications where mode-selection techniques are required.