In a nuclear emergency, protective actions for the public should be taken in time. It is internationally proposed that generic intervention levels (GILs) and generic action levels, determined based on cost-benefit analyses, be used as the decision criteria for protective actions. Operational intervention levels (OILs) are directly or easily measurable quantities corresponding to these generic levels. To assess the necessity of protective actions in a nuclear emergency, it is important that the environmental monitoring data required for applying and revising OILs should be promptly produced. It is discussed what and how to do for this task in the course of the emergency response. For an emergency environmental monitoring to be performed effectively, a thorough preparedness has to be made including maintenance of the organization and equipments, establishment of various procedure manuals, development of a supporting computer system and periodical training and exercises. It is pointed out that Korean legal provisions concerning GILs and OILs need to be amended or newly established.