This special issue brings together a selection of papers resulting from the STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project. STEMM-CCS was an ambitious, four-year, project on offshore geologic carbon dioxide storage funded under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 654,462). The aim was to deliver new insights, guidelines, and tools for the monitoring of CO2 storage at putative offshore Carbon dioxide Capture and Storage (CCS) sites. CCS is an important potential mitigation strategy to reduce anthropogenic CO2 emissions.