Marine microbes are uniquely important to life as we know it. Since life most likely began in the oceans, marine microorganisms are the closest living descendants of the original forms of life. They are also major pillars of the biosphere. Their unique metabolisms allow marine microbes to carry out many steps of the biogeochemical cycles that other organisms are unable to complete. The smooth functioning of these cycles is necessary for life to continue on earth. Early marine microorganisms also helped create the conditions under which subsequent life developed. More than two billion years ago, the generation of oxygen by photosynthetic marine microorganisms helped shape the chemical environment in which plants, animals, and all other life forms have evolved. A great deal of research on the biogeography of marine microorganisms has been carried out, but many unknowns persist, and more work is needed to elucidate and understand their complexity. It is now known that microorganisms live in every corner of the oceans. Their habitats are diverse and include open water, sediment, bodies of marine macro- and microorganisms, estuaries, and hydrothermal vents. By studying these habitats, scientists have developed a limited ability to predict the composition of marine microbial communities. It has also been found that some marine microbes have more cosmopolitan distributions than others. Recent work has found that most of the ecological principles that apply to larger organisms can also be applied to microorganisms, including marine microbes, but there are exceptions. Almost every ecophysiological parameter in the oceans is thought to have an impact on the diversity of microbial communities. Most of the direct interactions marine microorganisms have with larger organisms fall into one of two broad categories: symbiosis or pathogenesis. Beneficial microbial symbioses have enabled many invertebrate species to take advantage of habitats that would otherwise be unavailable to them. Invertebrates in these relationships may also enjoy the benefits of bioactive compounds microbes may produce to prevent bio-fouling or to ward off predators. Marine viruses are found in surprisingly high numbers in seawater, but it is likely that these populations are in equilibrium with their host populations. The metabolic diversity of marine microorganisms allows them to assume many roles in the biogeochemical cycles that other organisms cannot complete. Marine microbes are also able to adapt to the many extreme environments in the oceans. As humans continue to alter the environment, climate change will inevitably impact marine microbial communities and the biogeochemical cycles in which they participate, but the exact nature of these impacts cannot yet be predicted. Human health relies on a number of critical equilibria that marine microorganisms broker, including the balance between viruses and their hosts in the oceans, the balances that keep harmful algal blooms in check, the processes that control nutrient concentrations in marine waters, and others. The metabolic capabilities of marine microbes can be put to work in any number of biotechnology applications, including the manufacture of industrial products and energy production. Marine microbes are sources of novel bioactive compounds that may have application as pharmaceuticals. Potential applications for marine microorganisms in ameliorating environmental degradation also exist. Innovative approaches in research, education, and training are critical for moving the field of marine microbiology forward. Modern research in this field should embrace the new tools of genomics and metagenomics, but not to the exclusion of other methods of discovery. Education and training in marine microbiology needs to be multidisciplinary. Arrangements that expose graduate students and postdoctoral scientists to laboratories that do work outside the students' immediate fields of focus should be encouraged.