Silicon photonics (SiPh) has been established as the ubiquitous platform in the myriad of integrated photonics applications, from mainstream applications such as optical communications or microwave signal processing to emergent artificial intelligence and quantum processing. Most SiPh applications require optical phase shifters to vary the phase of the light with minimal optical loss. To date, SiPh phase shifters have been implemented mainly by leveraging the thermo-optic coefficient of silicon. The main benefits of thermo-optic phase shifters (TOPS) are the excellent fabrication compatibility with complementary metal-oxide-semiconductor (CMOS) technology and the possibility of achieving negligible optical loss. Therefore, TOPS are well-suited to provide scalability. However, the underlying heating mechanism makes TOPS power-hungry and slow for most applications. Here, we comprehensively review the main configurations and optimization strategies proposed to obtain energy-efficient and fast TOPS. Additionally, we compare TOPS with other electro-optic mechanisms and technologies that hold promise to provide a new generation of phase shifters in the SiPh platform.