In the case of a nuclear emergency situation, as compared to normal operation, other requirements such as type of information, availability in time and amount must be met in order to characterise the radiological situation and provide recommendations for countermeasures. These requirements are dependent upon the different phases of the emergency. In the beginning phases (Phase 1 and Phase 2) the timely availability of information is of great importance. The focus changes more and more to the diversity of the analysed environmental media in the course of the event. The status at the plant, source term estimates, meteorological data, and weather forecasts are needed in order to calculate trajectories and assessing the prognoses of the expected external dose and contamination during Phase 1 before the release. In addition, on-line dose rate measurements and information on the radionuclide spectrum are necessary. The aim is to get an overview of the actual radiological situation and its possible development in time in order to introduce countermeasures, where necessary. In Phase 2, when the dispersion of radioactivity is finished, measurements according to § 3 of the Precautionary Radiation Protection Act (StrVG) are of primary interest, particularly the determination of contamination to human food and animal feed. During the phase following the deposition, the external dose as a function of time can be well predicted if the nuclide spectrum is known. In Phase 3, i. e. weeks following the emergency, measurements are used to observe the activity concentrations in different media over a longer period of time.