The combination of MEMS fabrication with atomic clocks has enabled a new generation of small, cheap, low-power devices. Since the proposal of chip-scale atomic clocks (CSACs) in 2002, the fields of MEMS-based atomic sensors and coherent population trapping (CPT) spectroscopy have progressed rapidly. This review gives an overview over microfabrication techniques used for CSACs, the fabrication and integration of the critical components. Furthermore, the performance of MEMS clocks is evaluated in terms of frequency stability and sensitivity to external parameters, size, and power consumption. Possibilities for reaching the final goal of a MEMS clock with a fractional frequency instability below 10-11 at 1 hour of integration in a package of volume of 1 cm3, consuming 30 mW of power, are discussed. Finally, a broad field of advances spectroscopic techniques is reviewed and their advantages and drawbacks for the improvement of chip-scale atomic clocks.