Atrazine is an herbicide that has been used extensively throughout the U.S. for control of weeds in agricultural crops. It is a problem in the aquatic environment because it is toxic to aquatic organisms and is often found in surface water. EPA is developing ambient water quality criteria for atrazine through its authority under Section 304(a) of the Clean Water Act (CWA). These water quality criteria may be used by States and Tribes to establish water quality standards for atrazine. This document provides guidance to States and Tribes authorized to establish water quality standards under the Clean Water Act (CWA) to protect aquatic life from acute and chronic effects of atrazine. Under the CWA, States and Tribes are to establish water quality criteria to protect designated uses. While this document constitutes U.S. EPA's scientific recommendations regarding ambient concentrations of atrazine, this document does not substitute for the CWA or U.S. EPA's regulations; nor is a regulation itself. Thus, it cannot impose legally binding requirements on U.S. EPA, States, Tribes, or the regulated community, and it might not apply to a particular situation based upon the circumstances. State and Tribal decision-makers retain the discretion to adopt approaches on a case-by-case basis that differ from this guidance when appropriate. U.S. EPA may change this guidance in the future.