The accumulation of contaminants in organisms is crucial to environmental risk assessments. It depends on the chemical and physical properties of the contaminant and the ecosystem, the type of exposure (e.g. acute or chronic), and the efficiency with which organisms assimilate, metabolize, and eliminate the compound. To judge the severity of exposure and the efficacy of remediation, one must understand the complex dynamics of chemicals as they affect ecosystems. The sheer number of chemical components and the multitude, nonlinearity, and temporal dependence of the governing biological, chemical, physical, and geological processes and cycles make this a daunting task. Most research has focused on specific scenarios, emphasizing aquatic (freshwater and marine) ecosystems and either heavy metals or large organics. While much has been learned, generalizations of the results often fail, and a true understanding of the dynamics of contaminant accumulation has not yet been achieved.