The small size, ubiquity, metabolic versatility and flexibility, and genetic plasticity (horizontal transfer) of microbes allow them to tolerate and quickly adapt to unfavorable and/or changing environmental conditions. Prokaryotes are endowed with sophisticated cellular envelopes that contain molecules not found elsewhere in the biological world. Although prokaryotic cells lack the organelles that characterize their eukaryotic counterparts, their interiors are surprisingly complex. Prokaryotes sense their environment and respond as individual cells to specific environmental challenges; but prokaryotes also act cooperatively, displaying communal activities. In many microbial ecosystems, the functionally active unit is not a single species or population (clonal descendence of the same bacterium) but a consortium of two or more types of cells living in close symbiotic association. Only recently have we become aware that microbes are the basis for the functioning of the biosphere. Thus, we are at a unique time in the history of science, in which the interaction of technological advances and the exponential growth in our knowledge of the present microbial diversity will lead to significant advances not only in microbiology but also in biology and other sciences in general.