For many bacterial species, entry into mammalian cells is an important step toward establishing an infectious disease. Genetic and molecular techniques have revealed many important features of the entry process. As an example of this approach, the enteric pathogen Yersinia pseudotuberculosis has been used as a model system for bacterial penetration. This analysis has uncovered at least three different pathways for entry of the microorganism into cultured mammalian cells. These pathways differ in regards to their tissue specificities as well as the regulatory signals that control their expression. One of these pathways, promoted by the Y. pseudotuberculosis outer membrane protein invasin, has been studied in detail. This single factor is sufficient to promote entry of inert particles by binding multiple integrin receptors during cellular uptake. The significance of multiple pathways for entry as well as the binding of multiple receptors is discussed.