This paper describes the problem of maintaining cross-sectional representativeness in a longitudinal survey of a changing population. The extent and nature of the problem is outlined and potential solutions are described. The procedures adopted on Understanding Society are described. The main challenges are to correctly identify members of the initial sample who leave the population – through death or emigration – and to periodically add appropriate samples of people who join the population – through birth or immigration. The samples to be added should have known selection probabilities and should strike an appropriate balance between precision of estimation and cost-efficiency of fieldwork.