This paper aims to discuss complexity as a key feature for understanding the role of science knowledge in environmental and health contexts--a central issue in Science|Environment|Health pedagogy. Complex systems are, in principle, not predictable. In different contexts, ephemeral mechanisms produce different, sometimes completely unexpected results. The art of decision-making in complex contexts is to take scientific knowledge into account but to interpret its meaning in terms of concrete complex contexts. This is illustrated by four empirical studies on Science|Environment|Health issues, presented midway through this paper. The findings underscore the importance of introducing complexity issues into science education. Not only are all the grand health and environmental challenges of our times highly complex, but there is also evidence that introducing complexity into science education may support many students' motivation to learn science and change practice in science classrooms. Truly appreciating the role of complexity in Science|Environment|Health pedagogy is likely to raise future citizens who understand the delicate relation between predictability and adaption and to empower them for wise decisions about societal and personal well-being. [For the complete volume, "Bridging Research and Practice in Science Education: Selected Papers from the ESERA 2017 Conference. Contributions from Science Education Research. Volume 6," see ED615249.]