Microbes exist everywhere, and studies have demonstrated the presence of viable microorganisms in a crewed space habitat. Microorganisms within space habitats pose potential hazards to crew health and potentially damage hardware. Continuing successful long-term space habitation requires fundamental information on microbiological safety for avoiding biohazards in space. We discuss the importance of researching microbes in crewed space habitats and of monitoring microbes on the International Space Station (ISS). We also review environmental microbiology perspectives in crewed space habitats and the microbiology of the space environment (astromicrobiology).