This chapter discusses the interactions between heavy metal contaminants and soil solids, together with the various reactions in contaminant-particle interactions that contribute to the assimilative capacity of soils. These are prime mechanisms and processes that contribute to the attenuation of heavy metal contaminants during their transport in soils. For natural attenuation to be used as a tool in managing ground contamination by heavy metals, one needs to be concerned with the processes associated with partitioning and environmental mobility of these heavy metals. One needs to have knowledge of (a) the interactions between soil particles and heavy metals, and particularly of (b) processes that lead to partitioning. In addition, there are at least five factors one should consider in evaluating environmental mobility and bioavailability of heavy metal contaminants in the ground; these are: (1) hydrogeological setting, (2) changes in pH-regime of the affected ground, (3) changes in the ionic strength of the contaminated ground or soil-water system, (4) changes in the oxidation-reduction potential of the system, and (5) formation of complexes.