The European Marine Board Future Science Brief No. 13 ‘Monitoring, Reporting and Verification for marine Carbon Dioxide Removal' presents the key challenges for MRV, the status of the observing and modelling system that will be required to underpin MRV activities, and the status of existing protocols and governance frameworks addressing mCDR. The document then highlights that at present, given the early developmental stages of mCDR methods, none have sufficiently robust, comprehensive MRV in place to enable credible large-scale implementation. It closes by presenting recommendations for policymakers and regulators, science funders, and MRV scientists, practitioners and project planners, to bridge identified knowledge gaps and establish standardised MRV protocols for mCDR before they reach large-scale deployment.