The HIV/AIDS pandemic has challenged the resourcefulness of epidemiology and epidemiologists. In response to the challenge, epidemiologists have used existing epidemiologic strategies, expanded existing strategies, and developed new strategies to answer key questions about the transmission of HIV, the natural history of HIV at the molecular, host, and community levels, for evaluation of treatment effectiveness and intervention strategies, and to inform public health policy. In responding to the challenge of the pandemic, epidemiologists have also increasingly collaborated with scientists from other disciplines, particularly immunology, virology, and the behavioral sciences. Examples of the application of these epidemiologic strategies are presented.