This Data Explained summarises the content and potential limitations to users of the Welsh National Database for Substance Misuse (WNDSM) dataset. This is the primary dataset relating to individuals referred to, assessed and/or provided treatment within specialist substance misuse services in Wales. This paper aims to guide researchers who are interested in using the WNDSM dataset to investigate the role or impact of those engaged with substance misuse treatment services in Wales. This Data Explained has been produced by researchers working together within Public Health Wales, ADR Wales and SAIL team within Swansea University. The information presented in this Data Explained has been reviewed by Public Health Wales, ADR Wales and SAIL colleagues along with additional insights provided by WNDSM leads within Digital Health and Care Wales (DHCW) with expertise within this thematic area and is accepted to be Data Explained accurate at the point of publication. This Data Explained report presents the findings of a study conducted as part of the BOLD Substance Misuse Demonstrator Pilot (Phase 1) in Wales, which focuses on data quality of the WNDSM. Further information on the BOLD programme can be found here: Better Outcomes Through Linked Data (BOLD)