As key flow properties for coalbed methane production, greenhouse gas reduction through CO2 storage in coal and gas drainage to improve mining safety, knowledge of gas diffusion in coal has attracted significant interest. In the past decades, numerous efforts have been made to study the nature of gas diffusion in coal both experimentally and theoretically, and many models, including analytical and empirical models, have been proposed. However, because the assumptions, simplifications and applications of these models differ greatly, it is essential to critically review, compare and classify them for better use in practice. Therefore, this article reviews the state of the art on the gas diffusion in coal particles from the aspects of definition, mathematical models, experimental methods, and applications in environment, energy and mining safety areas. Moreover, this review article also dedicates to investigate the inter-relationships between different models, classify them into different categories and identify their limitations when they are applied in different areas. Based on these analyses, the potential research areas related to gas diffusion in coal are proposed.