High-resolution imaging of solute movement at pore scale in core samples and numerical simulations are presented to demonstrate the effects of pore-scale velocity variations, neglected in Darcy’s velocity, on the spread of solutes. Advection and dispersion equations (ADE) for solute transport in variably saturated media are thus formulated. Then, Peclet number analysis relates the dispersion coefficient to dispersivity --- the solute transport property of a porous medium -- and Darcy’s velocity. Well-controlled laboratory soil column experiments and related numerical experiments are examined to illustrate the validity and weakness of ADE. They also show that dispersion in porous media is an ensemble mean description of the effects of pore-scale velocity variations neglected by Darcy law. This chapter presents the dead-end pores, mobile-immobile zones, or dual-domain models, explains their ensemble mean nature, and discusses their pros and cons. Lastly, it formulates the ADE for the reactive solute in porous media.