This study investigated the use of covariation as a principle of causal analysis in children of 3-4, 6-7, and 9-11 years of age. The covariation principle specifies that an effect is attributed to the 1 of its possible causes with which it covaries. The results indicated that children as young as 3 years were capable of using covariation information in their attributions of simple physical effects. Children of 3-4 and 6-7 years had greater difficulty identifying inhibitory causes than in identifying facilitatory causes, while 9-11-year-olds identified the 2 types of causes with equal accuracy. 3-year-olds showed a tendency to attribute effects to causes that followed the effects, whereas older children appeared to assume that causes preceded effects.