SUMMARY: Advances in genetic and immunological approaches during the last few decades have transformed medicine and biomedical research. The human genome and the genomes of numerous model organisms are now fully sequenced. Initial exploitation of this wealth of genetic information has begun to revolutionize research on these species, and the appli- cations derived from it. Progress in understanding the ecology of microorganisms (including marine taxa) has followed closely on the heels of these advances, owing to the tremendous benefit afforded by major technological advances in bio- medicine. Through the application of these novel approaches and new technologies, marine microbial ecology has moved from a minor footnote within marine biology and biological oceanography during the 1950s and '60s to the focus of much of our present interest in the ocean. During the intervening half-century we have learned a great deal regarding the overall abundances, distributions and activities of microorganisms in the sea. Recognition of the extraordinary diversity of marine microbes, the predominant role that they play in global biogeochemical processes, and the potential for natural or engineered microbial products to benefit humankind, has placed marine microbes in the spotlight of both scientific and popular atten- tion. Our fascination with these minute denizens of the ocean is not likely to wane anytime soon. Recent studies have indi- cated that we still know relatively little about the breadth of microbial diversity in marine ecosystems. In addition, many (most?) of the predominant marine microbial forms in nature have not yet been brought into laboratory culture. Thus, our knowledge is still rudimentary with respect to the spectra of biochemical, physiological and behavioral abilities of these species, and the study of marine microbes will remain a major focus of investigations in marine science well into the fore- seeable future. As a large cadre of researchers moves headlong into this work, we can expect many new discoveries and more paradigm shifts regarding the composition and function of marine microbial communities.