Abstract The first step in the assessment of domino accident scenarios is the definition of the basic elements of escalation resulting in a domino effect. In the present chapter, the fundamental elements of a domino accident are discussed. A generic and unambiguous definition for “domino effect” is provided, taking into consideration a myriad of definitions for the term that have been suggested since 1984. An approach is elaborated and discussed to allow drafting a comprehensive and full-scale categorization of events that may constitute a domino accident. The concepts of propagation and escalation are clarified. The possibility of simple, multiple-level and parallel propagation are introduced and discussed. A framework for the quantitative assessment of domino scenarios is provided.