In this chapter, the basic principles of ecological risk assessment will be discussed as well as the context within which these assessments are used. Ecological risk assessment can be thought of as a logical process which estimates the likelihood that undesirable ecological effects may occur or are occurring as a result of human activities. These activities include the accidental or deliberate release of chemicals into the environment, physical disruptions such as construction, or the alteration of land for agricultural purposes. Although the process is not limited to chemicals, most risk assessments (human and ecological) within many state and federal agencies such as the U.S. Environmental Protection Agency have been oriented towards such substances. Since most of the experience lies with assessing the risks of man-made chemicals such as pesticides and a wide array of industrial chemicals, our discussion will focus on the concepts and procedures used to assess the ecological risks of man-made chemicals, or as they are commonly called xenobiotics. However, the basic concepts of ecological risk assessment are also applicable to nonchemical stressors such as physical habitat alterations and changes in environmental conditions such as water temperature and pH.