This chapter is the most crucial part of the book, the fundamental building block of the concept of dispersion in porous media and macrodispersion in the field-scale aquifers. It unravels the myth of macrodispersion, anomalous dispersion, scale-dependent dispersion, dual-domain, and other recently developed dispersion models for solute transport in aquifers (Chapters 9 and 10). This chapter first explains how the concept of dispersion evolves from molecular diffusion to account for the effects of fluid-dynamics-scale velocity variation in solute migration in a pipe. The relationship between dispersion and the concentration averaged over the cross-section of a pipe is visited. Further, this chapter illustrates the molecular and fluid-dynamics-scale velocity variations, the interaction between diffusion and dispersion, and scale issues associated with the shear flow dispersion. Finally, it discusses the limitations of extending Fick’s law for molecular-scale velocity variations to describe the effects of fluid-dynamics-scale velocity variations.