The chip?scale atomic clock technology based on coherent population trapping (CPT) atoms is reviewed, including the basic principle of CPT atomic clock, Ramsey?CPT atomic clock technology, schemes of CPT atomic clock suitable for miniaturization, and the development and current status of the chip?scale CPT atomic clock. The key technologies， such as laser frequency modulation, Ramsey technology, left and right circularly polarized light pumping (push?pull), laser and microwave frequency stabilization, and micro optical?electro?mechanical system (MOEMS), are analyzed and discussed. Finally, it is concluded that the CPT atomic clock is developing in the direction of low power consumption, chip?based and high clock precision.