Organizations are dynamic, hierarchically structured entities. Such dynamism is reflected in the emergence of significant events at every organizational level. Despite this fact, there has been relatively little discussion about how events become meaningful and come to impact organizations across space and time. We address this gap by developing event system theory, which suggests that events become salient when they are novel, disruptive, and critical (reflecting an event's strength). Importantly, events can originate at any hierarchical level and their effects can remain within that level or travel up or down throughout the organization, changing or creating new behaviors, features, and events. This impact can extend over time as events vary in duration and timing or as event strength evolves. Event system theory provides a needed shift in focus for organizational theory and research by developing specific propositions articulating the interplay among event strength and the spatial and temporal processes through which events come to influence organizations.