This article argues that an important source of the difficulties posed by particular concepts and theories is the narrow range of "types of causal models" with which most learners are familiar. Most learners are familiar with relatively simple styles of causal models, but many concepts and theories in science depend on styles substantially more complex. The authors offer an analysis of four dimensions of complex causality and argue that the increasingly complex styles along the dimensions present challenges that help to explain students' difficulties in mastering science concepts. Two types of evidence are offered: (1) analyses of students' understanding of several challenging science concepts based on the extant literature and research; and (2) two intervention studies are reviewed, involving teaching interventions where cultivating greater complexity in students' causal models led to better understanding. The interventions involve context-situated, inquiry-centered learning experiences that draw students' attention to how they are modelling the causality involved in particular phenomena and encourage more sophisticated causal modelling, embedded in their science learning. (Contains 3 tables and 3 figures.)