Under Section 1445 (a) (2) of SDWA, as amended in 1996, the Environmental Protection Agency (EPA) is required to establish criteria for a revised monitoring program for unregulated contaminants and, by August 11, 1999, to publish a list of contaminants to be monitored. To conform to the 1996 Amendments, EPA has proposed substantial revisions in its proposed Unregulated Contaminant Monitoring Regulation (UCMR), described in the UCMR Preamble and Rule (64 FR). The purpose of the UCMR Program is to collect unregulated contaminant occurrence data to help determine which of these contaminants EPA should regulate based on the concentrations of the contaminant(s) in public water systems and their potential adverse health effects through contaminant exposure. The purpose of this document is to describe the general statistical design, rationale, and specific methods used to select the representative sample of small systems that are required to undertake UCMR monitoring. Portions of this document also describe how this process relates to the UCMR monitoring plans of individual States.