Bilal M. Ayyub
This chapter focuses on defining and structuring systems for effective risk assessment. It presents multiple perspectives on system definition, emphasizing the importance of boundaries, interfaces, and hierarchical decomposition. Methods such as requirements analysis, functional analysis, work breakdown structures, decision trees, Bayesian networks, process models, state-based representations, and network models are introduced. The chapter highlights hierarchical definitions of systems and the role of knowledge and information structures in managing complexity. System complexity is examined in terms of interactions, dependencies, and emergent behavior. By providing a systematic approach to system definition and modeling, this chapter enables analysts to represent real-world engineering systems in forms suitable for quantitative reliability and risk analysis.