Idiopathic environmental intolerance (IEI) is a poorly understood condition that may involve disturbances in immunologic, neurologic, endocrine, behavioral, emotional, and cognitive processes. This chapter reviews theories and evidence that behavioral conditioning processes, including pharmacologic sensitization, conditioned immunomodulation, and conditioned odor and taste aversions, may play a role in the development and maintenance of IEI. It also reviews the psychophysiologic concepts of individual response specificity and situational response stereotypy as potential explanations for the individual differences observed in specific responses to environmental stimuli in patients with IEI. Finally, the treatment implications of a conditioning account of IEI are discussed as part of a more comprehensive treatment approach that incorporates other behavioral and nonbehavioral strategies.