Journal of Statistical Physics, Vol. 70, Nos. 1/2, 1993. p. 197-211.
We discuss some aspects of the intriguing problem of interplay between molecular diffusion and the geometry of the velocity field in the diffusion of test particles. By simple arguments one can understand how the diffusion coefficient can have a large enhancement from the combined effects of the noise and the drift terms in the Langevin equation ruling the motion of test particles. The same effects give rise to the superdiffusive transport observed in media with correlated random velocity fields.