Environmental impact calculations for sources of nuclear and conventional pollutants are usually based on the Gaussian equation for turbulent diffusion of waste air plumes in the atmosphere. Consequently, diffusion experiments deal with the determination of the diffusion parameters (Gaussian standard deviations) for different meteorological, topographical, and release conditions. The concepts and methods for the investigation of the diffusion parameters, particularly of the favorable tracer techniques and materials, were reviewed. The most important test series and the resulting systems of diffusion parameters were analyzed, and the new results of the Jülich experiments were compared with the systems of Pasquill, Klug, Brookhaven, and St. Louis. The comparison of the short- and long-time diffusion factors indicates that for the improvement of the diffusion calculations the underlying set of diffusion parameters has to be carefully selected considering the release height of the pollutants and the surface roughness of the local environment.