[en] An Operational Intervention Levels (OILs) is a type of action level that can be used immediately and directly (without further assessment) to determine the appropriate protective actions and other response actions on the basis of environmental measurements. This paper described the development of the concept of OILS and its evolvement as well as the default values of different versions and corresponding protective actions. Taking the Fukushima accident as an example, the differences of the earlier and late OILs versions in application of emergency actions were analyzed and compared. (authors)