Perceiving the affordance of a tool requires the integration of several complementary relationships among actor, tool, and target. Higher order affordance structures are introduced to deal with these forms of complex action from an ecological–realist point of view. The complexity of the higher order affordance structure was used to predict the difficulty of perceiving the tool function. Predictions were tested in 3 experiments involving children between 9 months and 4 years old. In a classical tool use task dating back to W. Köhler, a desirable target was obtained by using a hook as a tool. The relative positions of the hook and the target were systematically varied to obtain structures differing in complexity. The observed difficulty of the task was found essentially in accordance with the theoretical complexity of the higher order affordance structures involved in perceiving the tool function.