The ability of plants to remove pollutants from the environment is currently used in a simple and low-cost cleaning technology known as phytoremediation. Unfortunately, little is known about the metabolic pathways involved in the transformation of xenobiotic compounds and the ability of certain plants to tolerate, detoxify, and store high concentrations of heavy metals. Plant cell and tissue culture is considered an important tool for fundamental studies that provide information about the plant-contaminant relationships, help to predict plant responses to environmental contaminants, and improve the design of plants with enhanced characteristics for phytoremediation. Callus, cell suspensions, hairy roots, and shoot multiplication cultures are used to study the interactions between plants and pollutants under aseptic conditions. Many plant species have an inherent ability to accumulate/metabolize a variety of pollutants, but they normally produce little biomass. However, fast-growing trees are excellent candidates for phytoremediation because of their rapid growth, extensive root system, and high water uptake. This chapter outlines the in vitro plant production of both somaclonal variants and transgenic plants of Populus spp. that exhibit high tolerance to heavy metals.