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An on-chip frequency comb is a light source on a chip that generates coherent multifrequency light whose frequencies are equidistant in the optical frequency domain. It even suggests the possible realization of chip-scale devices with metrological precision in terms of time and frequency. In particular, a microresonator-based frequency comb is a novel on-chip frequency comb device that exhibits intriguing features as experimental platforms on which to explore unique nonlinear phenomena and tools for practical applications. In this chapter, we first provide an overview of conventional frequency combs and on-chip frequency combs. Then, we describe the fundamentals of microresonator-based frequency combs, their key features, and applications. Finally, we summarize the chapter by discussing various challenges and future perspectives in relation to this emerging on-chip frequency comb.