Public health officials have detected COVID-19 outbreaks through wastewater surveillance—testing for viruses that enter sewer systems in human waste. The technology has also been used to detect other pathogens and even chemicals, such as opioids. The recent pandemic spurred investment in wastewater surveillance. It could detect public health problems faster than individual testing—so why isn't it more widely used? We provide an overview of the challenges, which include: Standardizing the science to produce more useful data; Preventing privacy issues related to identifiable genetic data; Demonstrating the technology's value to support wider use.