In view of future manned space exploration Life Support Systems are mandatory for human survival in space. Late in 2017, a new regenerative Life Support System, named Advanced Closed Loop System (ACLS), shall be installed on board the International Space Station as Technology Demonstrator for future space missions. The important unit to close the regenerative process is the Carbon Dioxide Reprocessing Subsystem (CRA) processing H2 and CO2 to Water Vapour and CH4 by the means of the Sabatier Reaction. The supply of H2 is provided by the ACLS Subsystems OGA (Oxygen Generation Subsystem) and CO2 by the CCA (Carbon Dioxide Concentration Subsystem). The core of the CRA is a two-staged Sabatier Reactor and a water recovery unit downstream the Reactors to separate the water. The special design of the Reactor Assembly has been developed and improved since years assuring a high conversion rate of H2. The Reactor Assembly comprises the Reactor containing a certain catalyst and the thermal insulation to reduce the heat loss to a minimum. The first stage of the Reactor operates adiabatic at very high temperature to ensure a high reaction rate. The second stage, operating at a lower temperature compared to the first stage, is air cooled to maintain isothermal conditions thus increasing the conversion rate. This paper describes on the one hand in detail the development and improvements of the Sabatier Reactor Assembly over the years and presents the current hardware built status. On the other hand it describes the Sabatier process with test results gained during the stand-alone Subsystem Testing and the closed-loop operation ACLS System Testing on Engineering Model Level.