An Active Substance Master File (ASMF) can be used to replace the documentation for an active substance, required as part of an initial marketing authorisation application (MAA) or subsequent marketing authorisation variation (MAV). Appropriate guidance is given in the CHMP/CVMP guideline on active substance master file procedure. A specific ASMF can be used for multiple MAAs and/or MAVs in one or more Member States, which may or may not be connected through a European procedure. The same ASMF can be assessed by different Member States or Rapporteurs, which can result in duplication of work, inconsistent decisions being made on the same data and frequent and numerous updating of the ASMF (often at the request of the Competent Authorities). In order to harmonise assessment of ASMFs, reduce the frequent updates of ASMFs, and reduce the resource and regulatory burden on Competent Authorities, ASMF and MA holders, the Working Group on Active Substance Master File Procedures has established a worksharing procedure for the assessment of ASMFs, including a centralised EU numbering system for ASMFs and a centralised repository for the ASMF assessment reports. To ensure the future lifecycle of the ‘worksharing ASMF’ is captured, the scope of the worksharing procedure includes the submission of the initial ASMF and any subsequent updates, submitted as part of a new marketing authorisation application or variation through the main authorisation routes in Europe. Following a review of the experience gained post authorisation with EU/ASMFs accepted for use, the scope of the worksharing procedure may be extended.