The UK census authorities have proposed guidance for the 2021 census indicating that the sex question may be answered according to subjective gender identity. This raises issues about the measurement of sex and gender identity which other data collection exercises are also contending with. This paper addresses the questions that have arisen regarding the census, before going on to explain how these questions have emerged more widely. Finally, I address the difficulties of discussing the collection of data on sex in a climate of policy capture and silencing.