Issued primarily by COSPAR (the Committee On SPAce Research), international Planetary Protection Policies mandate that all spacecraft hardware in contact with extraterrestrial environments "of chemical evolution and/or origin of life interest and for which scientific opinion provides a significant chance of contamination which could compromise future investigations" (Kminek and Rummel, 2015) undergo biological burden control processes. These policies seek to limit the (forward) biological contamination of the target body by terrestrial microorganisms on the spacecraft, so that future missions to the target body will provide accurate and reliable scientific results. Also, these policies seek to prevent the (backward) biological contamination of the Earth by a sample returned from the target body. Bioburden reduction is an integral part of current space missions and its importance will magnify as bioburden requirements become more stringent in the future. Since life-detection and sample-return procedures require sterile handling in situ (to protect scientific results), subsystems and instruments which will be in contact with extraterrestrial matter must be sterilized to prevent a false positive. Since the first Viking mission, heat microbial reduction (HMR) has served as a well-understood common practice for reducing bioburden. More recently, NASA and ESA have approved a standard protocol for vapor hydrogen peroxide (VHP) microbial reduction to address some of the drawbacks of HMR by lowering operating costs and decreasing schedule impacts, as detailed in the certification processes conducted by NASA's Jet Propulsion Laboratory and Steris. Steris has also conducted many testing campaigns on behalf of JPL over the past 20 years. The main results of their campaigns are hence reported. However, even VHP has certain limitations that do not make it an all-encompassing microbial reduction/sterilization modality for spacecraft hardware. Therefore, this review also investigates the state-of-the-art sterilization and cleaning techniques used in other fields, such as in the medical, food, and drug industries, for application to flight hardware. Major techniques covered include cold atmospheric plasma, electron beam irradiation, and gamma irradiation. Some techniques have proven to be good candidates for adaptation for future NASA spacecraft missions. Techniques such as gamma irradiation (γ rad), can broaden the scope of NASA-approved protocols and expand the currently limited toolkit. Cleaning, the removal of bioburden, is also an important aspect of bioburden reduction; despite the best microbial reduction/sterilization technologies, dead microbes can interfere with and potentially invalidate the results of biosignature models of relevant celestial bodies. Therefore, cleaning techniques, such as carbon dioxide snow, can significantly contribute to the bioburden reduction process. With the development of standardized protocols for these additional microbial reduction/sterilization and cleaning modalities - in combination with the well-known techniques with NASA and ESA approved protocols - we anticipate that future space missions may be able to achieve a higher biological standard.