This chapter aims to provide the theory necessary to understand chemical extraction and soil washing. It provides a guideline for the design of chemical extraction or soiling washing unit operations. Key factors to consider in the design of chemical extraction or soil washing units are contaminant solubility in extracting fluid, gradation of soil particles, contaminant sorption characteristics, hydrogeology, and moisture content of the soil. Mass transfer fundamentals are applicable to soil washing and chemical extraction. The chapter presents the theories applicable to the design of soil washing process and chemical extraction processes (ex situ and in situ). Soil washing and chemical extraction have been used extensively in Europe and have had limited use in the United States. Chemical extraction and soil washing costs are most dependent on the post-treatment process. Equilibrium stage concepts and advective diffusive transport considerations are important fundamentals in the evaluation and application of chemical extraction processes.