This paper proposes an event-based approach with an intuitive simple graphical representation of the system and its environment for designing, analysis and testing safety-critical systems. The events are user actions and system responses, and are ordered according to the threats posed by the resulting system states. This ordering is an integral aspect of the graphical representation, making it possible to directly identify the risks associated with each and every functionally desirable, and undesirable, event relative to one another. Tests that target safety requirements are devised by examining possible traces of these events, represented compactly by regular expressions, exhibiting particular risk patterns such as human error and system failures.