Inspired by the success of semiconductor-chip-based MEMs rotation sensors, there has long been interest in the possible realization of chip-based optical gyroscopes. Such devices could potentially be lightweight and rugged, while featuring some of the sensitivity advantages of Sagnac-based sensing devices. However, the performance of integrated-optical gyroscopes has lagged behind MEMS devices on account of difficult-to-achieve requirements for low-optical-loss chip-based waveguides and optical resonators. Here, a chip-based ring laser gyroscope is described. Its sensitivity is high enough to measure the Earth's rotation. The physical principles of its operation and its current performance will be reviewed.