The Agency for Toxic Substances and Disease Registry’s (ATSDR) congressionally mandated Substance-Specific Applied Research Program (SSARP) consists of a research agenda for priority hazardous substances. In establishing the SSARP, ATSDR first identifies data needs in its Toxicological Profiles for these substances. The data needs are then subjected to further evaluation and prioritized using the Agency’s Decision Guide. The priority data needs represent essential information to improve the database for conducting public health assessments. The identified priority data needs reflect the opinion of ATSDR, in consultation with other federal programs. The needs identified here do not represent the priority data needs for any other agency or program.