Many in the system dynamics community argue that children are natural systems thinkers. Here we study how middle school students and teachers think about everyday settings involving feedback, stocks and flows, time delays and nonlinearities, prior to any formal training in these concepts. We develop instruments to elicit understanding of systems concepts and test them with students and teachers from two middle schools in the U.S.A. We find, with some exceptions, generally limited intuitive systems thinking abilities. “Open-loop” or one-way causal thinking is common. Explanations lack references to time horizons and time delays. Significant misconceptions of stock and flow structures appear regardless of age. Teachers generally outperformed students, although one-quarter of the students performed at the median level for teachers. We discuss the nature of students' and teachers' intuitive models of dynamic systems, explore potential barriers to understanding dynamic systems, and discuss implications for effective teaching of systems concepts. Copyright © 2007 John Wiley & Sons, Ltd.