Abstract Many coastal seas and inland waters have water properties which are outside the normal conditions used for ocean colour algorithm design. In the ESA-funded “Case-2 Extreme” project, retrieval methods for OC products in extreme Case-2 waters are developed, tested, implemented, and validated primarily with reference to the capabilities of the Sentinel-3 OLCI and SLSTR instruments but also tested for Sentinel 2 where applicable. Based on hyperspectral radiative transfer simulation, in situ, and Earth observation data, specific features of extreme scattering and absorbing waters are reviewed. A reflectance-based sub-classification scheme of Case-2 waters for various levels of turbidity is introduced. Opportunities of using new wavebands are highlighted.