Data on the sex industry is notably hard to obtain. Existing evidence points towards an increase in the number of people selling sex, particularly through the online industry. The growing and increasingly diverse population poses challenges to service provision, as new groups are less visible and less likely to be in contact with specialist services. Simultaneously, there are increased calls for policies regulating the sex industry to be grounded in evidence. Relying on systematic literature and data reviews, this article provides a synthesis of the evidence on the prevalence of sex work and prostitution in England and Wales. It shows that no existing source allows producing reliable estimates of the size and characteristics of sex markets. As a result, policy is informed by partial pictures. The article proposes local mapping, an underused approach, to inform both policy development and service provision.