Object use is a ubiquitous characteristic of the human species, and learning how objects function is a fundamental part of development. In this article the authors examine the role that intentionality plays in children's understanding of causal relationships during observational learning of object use. Children observed demonstrations in which causally irrelevant and causally relevant actions were performed to achieve a desired goal. The intentionality of these actions was manipulated using verbal markers. Irrelevant actions were performed either intentionally ("There!") or accidentally ("Whoops! I didn't mean to do that!"). Three-, 4-, and 5-year-olds, but not 2-year-olds, were less likely to imitate causally irrelevant actions performed accidentally than when they were performed intentionally. This suggests that older children used intentionality to guide causal inference and perceived intentional actions as causally effective and accidental actions as causally ineffective. Findings are discussed from an evolutionary perspective in relation to the cultural transmission of tool-use knowledge. (Contains 4 tables and 2 figures.)