Since early 2020, the UK has been carrying out wastewater monitoring for COVID-19. Wastewater samples are collected regularly across the country and analysed for SARS-CoV-2, the virus that causes COVID-19. Wastewater monitoring is part of monitoring systems to detect new COVID-19 outbreaks and support test and trace approaches. How can SARS-CoV-2 be detected in wastewater? How can wastewater monitoring be used as part of the response to the COVID-19 outbreak? And how are wastewater monitoring programmes being used across the UK and worldwide?