STEMM-CCS is an ambitious research and innovation project on geological carbon dioxide (CO2) storage that will deliver new insights, guidelines for best practice, and tools for all phases of the CO2 storage cycle at ocean Carbon Capture and Storage (CCS) sites. It brings together the main operator (Shell) of the world’s first commercial scale full-chain ocean demonstration CCS project (Peterhead Project) with the leading scientific and academic researchers in the field of ocean CCS. The work performed in STEMM-CCS will add value to this existing operational programme, and fill gaps in future capability by providing generically applicable definitive guides, technologies and techniques informing how to select a site for CCS operations, how to undertake a risk assessment, how best to monitor the operations, how to provide information on fluxes and quantification of any leakage; necessary for the European Union Emissions Trading Scheme (ETS) and to guide mitigation/remediation actions. All of this information will be used to better communicate the case for offshore CCS, with a particular focus on communities directly and indirectly impacted. During STEMM-CCS we will perform a simulated CO2 leak beneath the surface sediments at the site to be used for CCS as part of the Peterhead project. This experiment will be used to test CO2 leak detection, leak quantification, impact assessment, and mitigation/remediation decision support techniques currently at the Technology Readiness Level (TRL) stage 4-5 and support their development to a higher TRL. In addition, using new geophysical approaches STEMM-CCS will develop tools to assess leakage from natural geological features (e.g. chimneys) and engineered structures such as abandoned wells. The Peterhead project will commence during the life of STEMM-CCS and so a unique aspect is the focus on a real-world ocean CCS site covering its initial phases of implementation, with direct involvement of industrial partners.