Phytoremediation, the use of plant systems for contaminant cleanup, is among the fastest growing areas of environmental remediation research, technology development, and implementation. The practice is facilitated by a number of plant adaptations. First, selected plant species can take up and accumulate certain metals in their shoots at levels that are toxic to ordinary plants (hyperaccumulation). The phenomenon is an evolutionary adaptation of specific plant species to natural metal enrichments; consequently, its occurrence is geographically restricted, and the plant species involved are highly specific with respect to types of metals tolerated. Other plant species can take up, metabolize and thereby detoxify certain organic contaminants in their shoots, or cause their transformations through substances released into their root zones (rhizospheres). The extent to which an organic contaminant may be translocated in plants is determined largely by its partitioning into lipid phases (its lipophilicity), while rhizo-sphere transformations are determined largely by types and amounts of plant exudations, and microbial populations that are stimulated by these exudates. Interactions between plant systems and contaminants that can facilitate phytoremediation are complex, and complicated further by strong influences of climate, particularly temperature, and matrix factors such as nature and reactivity of surfaces, pH, and oxidation-reduction statuses. The ways in which the various factors interact to render a contaminant more or less susceptible to phytoremediation are discussed, with the objective of providing information needed for decision making when considering phytoremediation for the cleanup of particular contaminants.