Development of effective public health information systems requires understanding public health informatics (PHI), the systematic application of information and computer science and technology to public health practice, research, and learning. PHI is distinguished from other informatics specialties by its focus on prevention in populations, use of a wide range of interventions to achieve its goals, and the constraints of operating in a governmental context. The current need for PHI arises from dramatic improvements in information technology, new pressures on the public health system, and changes in medical care delivery. Application of PHI principles provides unprecedented opportunities to build healthier communities.