As the number of critical facilities such as data centers, Internet collocation facilities, hospitals, and air traffic control centers continues to grow, the sensitivity of expensive vibration sensitive equipment in these facilities to earthquake hazards is becoming a pressing concern. This paper addresses the seismic risk analysis of critical facilities by specifying the required reliability of components of vibration isolation systems for an improved performance and reliability of isolated sub-systems with respect to earthquake hazards. A four-story structure representing a critical facility with an isolation floor at the second floor level, carrying a computer rack, representing shock and vibration sensitive equipment, is considered in order to study the reliability requirements of isolation system components for the protection of the critical equipment from earthquake hazards. Including uncertainties such as isolation system characteristics, i.e. isolation stiffness and isolation damping, eccentricity in the superstructure, and ground motion characteristics along with 3D models subjected to bi-directional ground motions, allows us to represent the nature of the problem in a realistic way. Because of the complexity of the system and the analysis, and the difficulty of completely describing the probabilistic characteristics of the whole process analytically, the Monte Carlo simulation technique is used to determine probability distributions and failure probabilities. (c) 2005 Elsevier Ltd. All rights reserved.