Concrete objects are used to help children understand mathematical concepts. Research suggests that perceptually rich objects may hinder children's performance on mathematics tasks relative to bland objects. However, previous studies have confounded the perceptual richness of objects with children's established knowledge of the objects. The present study examined how these two factors influence children's developing counting skill. Children (M age = 3 yrs, 10 mo) were randomly assigned to counting tasks that used one of four types of objects in a 2 (perceptual richness: high or low) x 2 (established knowledge: high or low) factorial design. Results revealed an interaction between the two factors. These findings suggest that the use of perceptually rich objects can convey both advantages and disadvantages for children's performance on math tasks, depending on how the objects interface with children's established knowledge.