In July 2000, ORD's Ecological Risk Assessment Support Center received a request from the Ecological Risk Assessment Forum (ERAF) to provide insight to issues associated with site-specific ecological risk assessments involving populations. This request contained questions that have frequently been asked in the context of several regulatory programs, including Superfund, Resource Conservation and Recovery Act (RCRA), pesticide registration, water quality criteria and standards development, and National Pollutant Discharge Elimination System (NPDES) permitting, as well as in efforts undertaken by natural resource trustees. These questions include: - How should populations be defined. - What should we measure to characterize populations. -How do we make the connection between measurements made on individual organisms and population effects. - How can we measure and interpret effects on natural populations. - What spatial and temporal scales are appropriate when assessing risks to populations. This white paper attempts to support performance and interpretation of population-level risk assessments conducted during Superfund RI/FS and RCRA RFI/CMS processes by providing information relevant to the above questions.