Publisher Summary This chapter focuses on global scale dispersion, which is poorly known—whether predicted in advance or based on average climatology. There are many manmade and natural substances in the atmosphere that might be lent to regional and global scale dispersion studies, but, unfortunately, most of these are subject to significant removal processes. Model simulations of global scale dispersion fall into two categories depending on the degree of parameterization of turbulent transport. The first model parameterizes all turbulent motions allowing for tracer transport in the usual sense only by organized circulations. The second employs three-dimensional winds at a series of grid points over the globe to transport tracer substances on a synoptic and global scale. Because the verification of any dispersion model lies at the heart of its validity, the data on tracer substances are reviewed in the chapter and a semiempirical model is suggested. The model is then compared with the limited amount of verifying data on tracer substances. Viewed as an engineering approach to the prediction of inert tracers injected on a global scale, the coarse features of the distribution are reasonably well approximated in the troposphere by a flux-gradient model and a prescribed set of diffusion coefficients.