Bio-behavioural surveys of people who inject drugs (PWID) evolved from unlinked anonymous monitoring (UAM) of human immunodeficiency virus (HIV) incidence and prevalence, which began in some high-income countries in the late 1980s. UAM was conducted purely for surveillance purposes and test results were not returned to participants. Later, the importance of collecting data on behavioural risk factors was recognised, leading to the development of bio-behavioural surveys of PWID, which today are conducted regularly in several countries. Typically, these surveys recruit participants from venues providing harm reduction services and involve behavioural questionnaires and dried blood spot (DBS) testing for HIV and hepatitis C (HCV). DBS test results are not returned to participants; instead, countries offer varied systems of on-site testing separate from the bio-behavioural testing or provide referrals to external testing services. In this commentary, we trace the history of bio-behavioural surveys of PWID from their origins to the present day to explain how the methodologies evolved, along with the ethical considerations underlying them. We highlight the dramatic improvements in treatments for HIV and HCV over the past thirty years and the corresponding need to ensure that bio-behavioural survey participants can access low-barrier and timely testing. We review the pros and cons of different strategies for providing test results to participants and argue that the return of DBS results collected as part of bio-behavioural surveys warrants consideration as an additional tool to improve testing access for participants. Any changes should be informed by the perspectives of participants, study site personnel and investigators.Copyright © 2023. Published by Elsevier B.V.