The Algorithm Theoretical Baseline Document (ATBD) provides a detailed description of the algorithms that are used within the ESA CCI Soil Moisture production system. The ESA CCI SM production system was initially developed within CCI Phases 1 & 2 and is continuously being updated within CCI+ to reflect the current state of the science driving the system. The aim of this document is to describe the algorithm development process for each of the ESA CCI SM products, as well as provide an executive summary setting them within framework for the CCI project and the ESA CCI SM production system. This document forms the deliverable D2.4 Algorithm Theoretical Basis Document (ATBD), version 1.0, and was compiled for the European Space Agency (ESA) Climate Change Initiative Plus Soil Moisture Project (CCN 3 to ESRIN Contract No: 4000126684/19/I-NB” ESA CCI+ Phase 1 New R&D on CCI ECVS Soil Moisture”). This document supports CCI SM product version v08.1