Phase modulation technology is widely used in detecting the rotational signal of gyro in resonator integrated optic gyroscopes in order to improve the sensitivity and suppress noises. This paper we review various phase modulation techniques that have been proposed by many researchers in recent years. The influences of backscattering noises on the performance of RIOG are introduced firstly. Various improved phase modulation techniques are proposed by different research groups. The advantages and limitations of these modulation techniques are investigated. The modulation techniques include two broad categories: namely, single-phase modulation technique (SPMT) and double phase modulation technique (DPMT). Compared with SPMT, DPMT can further improve accuracy and system robustness of RIOGs. High precision sideband locking technology is the latest emerging modulation method that is expected to fulfill performance requirements in the fields of aerospace and defense.