The purpose of this chapter is to present and discuss the various kinds of inorganic- and organic-type contaminants that find their way on and into the ground, and the use of the knowledge-based passive remediation techniques of contaminated sites/ground using the natural attenuation capability of the subsoil known as monitored natural attenuation (MNA). Natural attenuation refers to the situation when attenuation of contaminants results because of the processes that involve the natural assimilative capacity of soil. Reliance is placed on the natural processes occurring in the soil that serve to reduce the toxicity of the contaminants and/or the concentration of the contaminants. These natural processes of contaminant attenuation include dilution, partitioning of contaminants, and transformations. They involve a range of physical actions, chemical and biologically mediated reactions, and combinations of all of these. With proper design procedures and knowledge of the interactions that can be favorably achieved to produce partitioning of contaminants, it is possible to structure a contaminated site management program that would reduce or even eliminate health threats to biotic receptors and damage to the environment. Detailed protocols and guidelines, which can be site-specific or generic, are designed to cover the essential requirements, procedures, standards, criteria, etc. that need to be used in determining the multiple lines of evidence and the effectiveness of the MNA process.