Because of the potential for microbes to cause infection in the astronauts and to be able to degrade various components that may be vital for the functioning of the spacecraft, the risks must be assessed and, where appropriate, manage the levels of microbial growth controlled by the use of good astronautical hygiene. For example, by frequently sampling the space-cabin air and surfaces to detect early signs of a rise in microbial contamination, keeping surfaces clean by the use of disinfected clothes, by ensuring that all equipment is well maintained in particular the life support systems and by regular vacuuming of the spacecraft to remove dust etc. It is likely that during the first manned missions to Mars that the risks from microbial contamination could be underestimated unless the principles of good astronautical hygiene practice are applied. Further research in this field is therefore needed so that the risks of exposure can be evaluated and the necessary measures to mitigate microbial growth are developed.