The term behavior engineering (BE) encompasses a broad integration of behavioral and compositional requirements needed to model large-scale systems. BE forms a connection between systems-engineering processes and software-engineering processes. In software engineering, interpreting requirements can be perceived as specifying behavior, which is viewed in terms of chronology of events in the modeled system. In this paper, we adopt BE in its general and integrating sense to search for a unifying notion of an event as a fundamental behavior-modeling concept. We examine several bodies of research with various definitions of an event and its basic units and structure. We use the thinging machine (TM) model to analyze notions related to events, including Dromey s behavior trees, fluents (change over time), recurrent events, and Davidson s events. The results point to an underlying meaning that can lead to a unifying event concept.