In the process of reviewing toxicology testing paradigms currently in use within the Agency for hazard characterization, the Risk Assessment Forum's Reference Dose (RfD) Technical Panel identified aging as an area needing additional consideration within the risk assessment process. The need to consider the aged or elderly as a special subpopulation when assessing risk to human health is supported by a number of considerations, e.g., the potential for the elderly to metabolize environmental agents differently than younger adults, and the proposal that environmental exposures may be related to the onset of certain diseases of the elderly (e.g., Alzheimer's and Parkinson's diseases). This document, developed by Versar, Inc., summarizes the current knowledge on the changes occurring in the aged and how these changes alter, or have the potential to alter the response to a toxicant. Following a discussion of issues which define the aged, the authors address the various biological changes that occur during the aging process. The authors conclude the document with a discussion on the impact these aging changes have on the human response to environmental challenges.