The Centers for Disease Control and Prevention (CDC) began wastewater monitoring in September 2020 to pro-vide additional information to healthcare providers and the public on thelevel of transmission of severe acute respiratory syndrome coronavirus 2(SARS-CoV-2), the virus that causes COVID-19, and to inform public healthactions. This wastewater monitoring rapidly grew from a small number ofstates to a system in all 50 states that includes approximately 40% of the US population. Wastewater monitoring complements traditional clinical monitoring systems by detecting viruses being shed from infected individuals that might otherwise be missed; infections from asymptomatic individuals and those who do not seek or have access to clinical testing or care might be captured in wastewater-monitoring data from a given sewer shed. Wastewater-monitoring data have been particularly valuable for managing COVID-19 because of the high rate of asymptomatic infections in humans contributing to SARS-CoV-2 transmission. Use of SARS-CoV-2 data from wastewater testing has been linked to successful public health actions,such as increasing uptake of vaccines through positioning of and communication about vaccination clinics. The CDCand other public health and academicpartners have expanded wastewater monitoring in the United States and globally to other pathogens, including in?uenza A viruses. Although both in?uenza viruses and coronaviruses have zoonotic hosts, the animal hosts for SARS-CoV-2 appear signi?cantly less likely to contribute to wastewater than do the dairy cattle and milk thatare central to the current outbreak ofhighly pathogenic avian in?uenza (HPAI)A(H5N1) virus in animals. In the context of the current zoonotic outbreak of HPAI A(H5N1) virus in livestock, the CDC is monitoring for human infections using multiple systems, including public health laboratory testing to track in?uenza viruses such as novel viruses, trends in clinical laboratory data for in?uenza, and emergency department visits potentially associated with in?uenza. There have been 14 instances of avian in?uenza A(H5) virus infection in people in the United States since 2022, of which occurred between January and August 2024; these have been among people exposed to either infected poultry or infected dairy cattle. To use wastewater monitoring to complement traditional monitoring systems, it is urgent to better understand the meaning of in?uenza A virus levels and the H5 subtype in wastewater to inform public health emergency response action. In?uenza A virus testing is being conducted on samples from more than 700 wastewater sites across 48 states,with testing occurring one to three times per week and data reported to CDC on a weekly basis. The CDC began publicly reporting wastewater data for in?uenza A viruses in May 2024 with an interim measure comparing the current