Microbial pathogenicity is a complex phenomenon encompassing many varied mechanisms. There are, however, several common strategies that pathogenic organisms use to sustain themselves and overcome host barriers. In one of these strategies, the first step is the firm adhesion of the microorganism to host cells. Adhesion is frequently mediated by organelles called fimbriae. A surprising variety of fimbrial adhesins have been found in Escherichia coli, and several have been extensively characterized. The genetics and regulation of the various fimbrial structures are reviewed, and strategies for preventing the adhesion of harmful bacteria to host surfaces are discussed.