As none of the concepts used for defining critical infrastructures over the years are considered comprehensive, in this paper we propose a method for defining them using a set of criteria and a weighted scoring technique. This set comprises criteria developed from Normal Accidents Theory and High Reliability Theory which is enhanced with several other criteria selected from various studies and official documents. The contribution brought by this method for determining what might be the appropriate criteria to be used for defining an infrastructure as critical, hopefully will become generally accepted. For demonstration, a nuclear power plant was tested, and the results show that it is a critical infrastructure, a conclusion perceptibly expected. (C) 2011 Elsevier Ltd. All rights reserved.