With the beginning of the year 2004, new case definitions of the Robert Koch Institute (RKI) replace those introduced in 2001. The new edition provides definitions for 53 categories of cases that are to be reported by local health departments through state health authorities to the RKI according to the German infectious disease control act. The new case definitions take into account recent developments in laboratory methods and indications for improvement identified through systematic evaluation. The case definitions consist of three types of evidence-clinical picture, laboratory detection, and epidemiological confirmation-all of which list disease- or pathogen-specific test criteria. Combining the three evidence types results in five case definition categories: clinically diagnosed illness, clinically and epidemiologically confirmed illness, clinically and laboratory-confirmed illness, laboratory-detected infection not fulfilling clinical criteria, and laboratory-detected infection with unknown clinical picture.