Cumulative effect assessments (CEAs) are needed to inform environmental policy and guide ecosystem-based management for marine ecosystems. This new paper, published in Science of the Total Environment, is based upon work from COST Action 15121 ‘Advancing marine conservation in the European and contiguous seas (MarCons). We provide guidance to operationalize a risk-based approach to CEAs by describing for each step guiding principles and desired outcomes, scientific challenges and practical solutions.