This article reviews the principles and applications of passive spontaneous emission spectroscopy (SES) for the quantitative determination of alkali metal concentrations emitted from combustion processes. The combustion of fuels that contain a high alkali metal content (Na and/or K) is challenging, as alkali metals reduce the slag formation temperature and induce fouling, causing combustion facilities to shutdown prematurely. The in situ on-line quantification of alkali metals is, thus, a critical measure to control combustion processes, preventing slagging and fouling from occurring. This review shows that several SES systems, developed by the Huazhong University of Science and Technology (HUST), are inexpensive, portable, and useful for measuring the alkali metal content, and have been applied for biomass combustion as well as coal and municipal solid waste combustion, from laboratory-scale settings (20 kW) to industrial facilities (300 MW). Compared with other research, the SES system from HUST has successfully quantified the emitted alkali metal concentrations during combustion. This review also highlights the challenges of the SES system and recommends further work to improve it for further applications.