An epidemic is an excessive occurrence of a disease, compared to what is usually observed, in a specific location and over a specific period of time. The occurrence of an epidemic is not a matter of chance. The detection, investigation, and control of epidemics are three closely interrelated functions, the ultimate goal of which is public health action and prevention. Field investigation relies on a multidisciplinary approach (clinical, epidemiological, environmental, and microbiological) with a descriptive and often analytical component (hypothesis testing). We present the methodological approach illustrated by examples of investigations into community-acquired and nosocomial outbreaks. The steps of an investigation are: confirming the existence of the outbreak; defining the disease (cases); identifying cases; describing cases in terms of temporal (epidemic curve), spatial, and personal characteristics; formulating hypotheses regarding the mode of transmission; testing these hypotheses; conducting an appropriate environmental investigation; conducting the necessary microbiological analyses; implementing control and prevention measures; and drafting an investigation report to share the lessons learned with the public health and scientific communities. An outbreak investigation is an iterative process, with the information gathered at each stage guiding the next in a deductive manner. Outbreak investigations often take place within a complex media, social, institutional, and political context that must be taken into account. The earlier the outbreak is detected and the investigation conducted promptly in collaboration with health authorities, the greater the potential preventive impact of the measures implemented.