This paper discusses the various attempts along the lines of statistical mechanics that have been made to explain certain phenomena occurring in flow of fluids through porous media. The latter particularly concerns processes of mixing miscible phases during their passage through a porous medium. The statistical theories that can be proposed are based on the following theoretical models: random walk of individual fluid particles, quasi‐turbulent flow analogy, analogy with thermodynamics, and finally, general statistical mechanics. In all cases, one ends up with equations that are very similar to a diffusivity equation.