Abstract: Infants' developing causal expectations for the outcome of a simple tool-use event from ages 8 to 12 months were investigated. Causal expectations were studied by comparing infants' developing tool-use actions (i.e., as tool-use agents) with their developing perceptual reactions (i.e., as tool-use observers) to possible and impossible tool-use events. In Experiment 1, tool-use actions were studied by presenting infants, ages 8 and 12 months, with tool-use object-retrieval problems. In Experiment 2, a second age-matched sample of infants watched a comparable series of possible and impossible tool-use events in which a tool was used to retrieve a goal-object. Two core related findings were made. First, infants' causal action and causal perception develop in parallel. In both action and perception, supporting tool-use develops before surrounding tool-use. Second, infants' tool-use action develops before their causal perception of comparable tool-use events. The findings support the constructivist hypothesis that infants' causal actions may develop before and inform their causal perceptions.