Determinism is frequently understood as implying the possibility of perfect prediction. This possibility then functions as an assumption in the Manipulation Argument for the incompatibility of free will and determinism. Yet this assumption is mistaken. As a result, arguments that rely on it fail to show that determinism would rule out human free will. We explain why determinism does not imply the possibility of perfect prediction in any world with laws of nature like ours, since it would be impossible for (...) an agent to predict with certainty any future event that is causally influenced by events outside her own backward light cone yet inside the backward light cone of the future event. This is the _light-cone limit_ and it undermines the Manipulation Argument or limits what this argument can tell us about the relevance of determinism to free will. We also respond to objections that the light-cone limit is irrelevant to the Manipulation Argument. (shrink)