Abstract: Total electron content (TEC) of the ionosphere is a critical parameter affecting the performance of radio systems. Leveraging the advantage of Global Navigation Satellite System(GNSS)with the global coverage and real-time capabilities, GNSS-TEC has become a vital tool for ionospheric observation and research, though its accuracy remains constrained by challenges such as hardware biases and the ionospheric spatiotemporal complexity. Based on ionospheric radio wave propagation theory, this paper elaborates on the fundamental principles of GNSS-TEC derivation, systematically categorizes common derivation methodologies, and reveals that its core lies in the synergistic optimization of ionospheric model assumptions, mathematical modeling between ionospheric TEC and GNSS observables, and solution algorithms. By analyzing the characteristics of GNSS-TEC retrieval methods across different periods, this study identifies that advancements in GNSS-TEC inversion methods are jointly driven by scientific demands, engineering applications, GNSS technological evolution, and computational power enhancement. Finally, the paper provides prospects for future trends in the development of GNSS-TEC derivation techniques.