This document provides the theoretical basis that hides behind the modules of MAJA processor. MAJA stands for MACCS-ATCOR Joint Algorithm, where MACCS was the Multi-Temporal Atmospheric Correction and Cloud Screening software, developed by CNES and CESBIO, and ATCOR is the Atmospheric Correction soft- ware developed by DLR. MAJA is based on MACCS architecture and includes modules that come from AT- COR. The MAJA processor is applicable to time series of Sentinel-2, Landsat, Venµs, and Formosat satellites. This Algorithmic Theoretical Basis Document (ATBD) provides a scientific description of the methods used within MAJA and some justifications of the choices made, as well as basic validation results.