The Office for National Statistics (ONS) is exploring models for the future of population statistics. The UK does not have a population register or a set of coherent identifiers across administrative datasets held by government. The current population statistics system is underpinned by the decennial Census, which is expensive and is arguably becoming increasingly unwieldy as a source of data in a rapidly evolving society and with ever increasing demands for more timely, relevant statistics. The system is also highly reliant on a port-based survey to measure migrant flows to and from UK, with the result that the intercensal population size estimates tend to have an increasing element of bias. The ONS is therefore researching how it can transform its population statistics system within that context. This paper outlines the current plans for this, and focuses on some of the methodological challenges underpinning the transformation.