AbstractThe Ocean Colour Climate Change Initiative (OCCCI)providesalong-termtimeseriesofoceancolour data and investigates the detectable climate impact. A reliable and stable atmospheric correction (AC) procedure is the basis for ocean colour products of the necessary high quality. The selection of atmospheric correction processors is repeated regularly based on a round robin exercise, at the latest when a revised production and release of the OC-CCI merged product is scheduled. Most of the AC processors are under constant development and changes are implemented to improve the quality of satellite-derived retrievals of remote sensing reﬂectances. The changes between versions of the inter-comparison are not restricted to the implementation of AC processors. There are activities to improve the quality ﬂagging for some processors, and the system vicarious calibration for AC algorithms in their sensor speciﬁc behaviour are widely studied. Each inter-comparison starts with an updated in-situ database, as more spectra are included inordertobroadenthetemporalandspatialrangeof satellite match-ups. While the OC-CCI’s focus has laid on case-1 waters in the past, it has expanded to the retrieval of case-2 products now. In light of this goal, new bidirectional correction procedures (normalisation) for the remote sensing spectra have been introduced. As in-situ measurements are not always available at the satellite sensor speciﬁc central wavelengths, a band-shift algorithm has to be applied to the dataset.