ECSS-E-ST-35-06 belongs to the Propulsion field of the mechanical discipline, and concerns itself with the cleanliness of propulsion components, sub-systems and systems The standard •defines design requirements which allow for cleaning of propulsion components sub-systems and systems and which avoid generation or unwanted collection of contamination,•identifies cleanliness requirements (e.g. which particle / impurity / wetness level can be tolerated),•defines requirements on cleaning to comply with the cleanliness level requirements, and the requirements on verification,•identifies the cleanliness approach, cleaning requirements, (e.g. what needs to be done to ensure the tolerable level is not exceeded, compatibility requirements),•identifies, specifies and defines the requirements regarding conditions under which cleaning or cleanliness verification takes place (e.g. compatibility, check after environmental test).The standard is applicable to the most commonly used propulsion systems and their related storable propellant combinations: Hydrazine (N2H4), Mono Methyl Hydrazine (CH3N2H3), MON (Mixed Oxides of Nitrogen), Nitrogen (N2), Helium (He), Propane (C3H8), Butane (C4H10) and Xenon (Xe).This standard is the basis for the European spacecraft and spacecraft propulsion industry to define, achieve and verify the required cleanliness levels in spacecraft propulsion systems.This standard is particularly applicable to spacecraft propulsion as used for satellites and (manned) spacecraft and any of such projects including its ground support equipment.External cleanliness requirements, e.g. outside of tanks, piping and aspects such as fungus and outgassing are covered by ECSS-Q-ST-70-01.This standard may be tailored for the specific characteristic and constraints of a space project in conformance with ECSS-S-ST-00.