In spite of the fact that the concept of safety barriers is applied in practice, discussed in the literature, and even required in legislation and standards, no common terminology that is applicable across sectors have been developed of the concept of safety barriers. This paper focuses on safety barriers and addresses the following aspects; definitions and understanding of what is a safety barrier, classification of safety barriers, and attributes of importance for the performance of safety barriers. Safety barriers are physical or non-physical means planned to prevent, control, or mitigate undesired events or accidents. Barrier systems may be classified according to several dimensions, for example as passive or active barrier systems, and as physical, technical, or human/operational barrier systems. Several attributes are necessary to include in order to characterize the performance of safety barriers; functionality/effectiveness, reliability/availability, response time, robustness, and finally a description of the triggering event or condition. For some types of barriers, not all the attributes are relevant or necessary in order to describe the barrier performance. (C) 2006 Elsevier Ltd. All rights reserved.