We briefly overview the theoretical advances for the fundamental description on the supercritical fluids since the late 1980s. The concept of supercritical gas-liquid boundary and its importance are introduced. The current state of the state-of-art theories about supercritical gas-liquid boundary (Integral Equation Approximation, Fluctuation Theory of Solutions, and molecular simulations) are examined. Based on these principles and the remarkable progresses in the field of supercritical fluids, the direction of future research and promising methodologies are given. The future researches on the fundamental descriptions of supercritical fluids are expected to play a crucial role in extending the knowledges and utilizing supercritical fluid technologies.