“Emergence” of infectious disease agents in humans, domestic animals and wildlife during the past 20 years has been widely described. Perception of emergence derives largely from the application of improved identification methods, including refinements in molecular, serologic, and microscopy technologies that enable researchers to monitor species migration with greater sensitivity. The reality of emergence comes from shifts in genetic profiles and from influences of ecological changes often brought about by human interventions designed for economic or quality-of-life gains, and ecologic management. CDC has frequent involvement with many outbreaks caused by various infectious disease agents, some of which may be considered emerging. Notable unusual agents include filoviruses (Fig. 1) and hantaviruses (Fig. 2). More typically encountered agents include caliciviruses (Fig. 3) and influenza viruses (Fig. 4). Recently, threats of national and international bioterrorism have added to CDC's responsibilities for prompt identification of infectious agents.