Through the application of molecular and cellular microbiology, we now recognize the diversity and dominance of microbial life forms on our planet, which exist in all environments. These microbes have many important planetary roles, but for us humans a major problem is their ability to colonize our tissues and cause disease. The same techniques of molecular and cellular microbiology have been applied to the problems of human and animal infection during the past two decades and have proved to be immensely powerful tools in elucidating how microorganisms cause human pathology.