Purpose: This inquiry is the first comprehensive, empirical analysis of the nature and measurement of flow in elementary teachers. The clearest sign of flow is the merging of action and awareness, that is, the degree to which an activity becomes spontaneous and automatic and individuals lose conscious awareness of themselves as they perform a task such as teaching. The basic objective of the research was to examine the theoretical structure and measurement of flow in elementary teachers. Research Methods: A typical sample of 260 elementary teachers from rural, urban, and suburban elementary schools in Ohio was used to test two rival explanations about the nature of flow. Structural equation modeling was used to assess the goodness of fit of the two models. Findings: Two rival explanations of flow, the Jackson-Marsh and the Quinn models, were evaluated using confirmatory factor analysis and structural equation modeling. The Jackson-Marsh model of flow proved to be a better explanation of flow of elementary teachers than the Quinn model. Consistent with Csikszentmihalyi’s explanation of flow, the construct consists of nine elements, all of which form an integrated whole. In addition, and as predicted, optimism was positively related to flow. Discussion: Because most flow research has been in sports and leisure, Quinn’s research in knowledge work seemed especially relevant for the study of flow in teachers. In spite of that fact, the Jackson and Marsh model was the better fit; the Quinn model provided insight into the dynamics of flow in elementary schools. Finally, although optimism was positively related to flow, it was academic optimism, not dispositional optimism, that was a strong predictor of flow in elementary teachers. Conclusions: The nature of flow in schools is instructive, offering insight into the elements of flow and how they collectively and individually inform us in pursuit of optimal teaching and learning conditions, but much more research remains to be done.