This document provides the theoretical basis for the Sentinel-3 OLCI IdePix. IdePix stands for Identification of Pixel properties and is a multi-sensor pixel identification tool available as a SNAP (Sentinel Application Platform) plugin. IdePix provides pixel identification algorithms for a wide variety of sensors like Sentinel-2 MSI, Sentinel-3 OLCI, MERIS, Landsat-8, MODIS, VIIRS, Proba-V or SPOT VGT. IdePix for Sentinel-3 OLCI classifies pixels of L1b products into a series of categories (flags) for further processing using a mono-temporal approach and background information. The Sentinel-3 OLCI IdePix derives cloud flags on two confidence levels, as well as cloud shadow, mountain shadow, snow/ice and water flags. This Algorithmic Theoretical Basis Document (ATBD) provides a scientific description of the methods used for the Sentinel-3 OLCI IdePix