A systematic approach to the assessment of thermal risks linked with the performance of exothermal reactions at industrial scale was proposed a long time ago. The approach consisted of a runaway scenario starting from a cooling failure and a classification of these scenarios into criticality classes. in the mean time these tools became quite popular and many chemical companies use them. Recently, the international standard IEC 61511 required the use of protection systems with reliability depending on the risk level. Since the criticality classes were developed as a tool for the choice of risk reducing measures as a function of the criticality, it seems obvious that the criticality classes may be used in the context of the standard IEC 61511, which provides a relation between the risk level and the reliability of protection systems. Firstly, the runaway scenario and the criticality classes will be shortly described. Secondly, the assessment criteria for severity and probability of occurrence of a runaway scenario will be described together with the required data and their inter-pretation in terms of risk. Thirdly, the assessment procedure is exemplified for the different criticality classes. Finally, the design of protection measures against runaway and the required IPL and SIL are based on the risk assessment obtained from the criticality classes. This approach allows minimising the required data set for the safety assessment and for the definition of the protection system designed in order to avoid the development of the runaway. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.