This paper outlines the basic principles of two?photon transition, and then introduces the experimental progress of two?photon optical frequency standards excited by continuous?wave and pulsed laser. Developing the lasers with higher power and lower frequency noise to suppress shot noise and intermodulation noise is the most effective way to improve the short?term stability of two?photon optical frequency standards which is limited by the performance of local oscillators. Finally, this paper summarizes the applications of two?photon optical frequency standards in science and engineering. The measurement of energy level spacing of hydrogen based on two?photon direct comb spectroscopy has become one of the most important techniques to measure Rydberg constant and proton charge radius, and plays an essential role in the test of quantum electrodynamics. The combination of two?photon optical frequency standard and microresonator?based optical frequency comb will pave the way for the miniaturization and integration of optical atomic clocks.