Abstract: Total Electron Content (TEC) of the ionosphere is a critical parameter affecting the performance of radio systems. Leveraging the advantage of 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 ionosphere’s 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. Finally, the study highlights that advancements in GNSS-TEC inversion methods are jointly driven by scientific demands, engineering applications, GNSS technological evolution, and computational power enhancement. Future demands in real-time performance and accuracy will further propel the evolution of GNSS-TEC derivation techniques.