Do citizens hold their representatives accountable for policy decisions, as commonly assumed in theories of legislative politics? Previous research has failed to yield clear evidence on this question for two reasons: measurement error arising from noncomparable indicators of legislators’ and constituents’ preferences and potential simultaneity between constituents’ beliefs about and approval of their representatives. Two new national surveys address the measurement problem directly by asking respondents how they would vote and how they think their representatives voted on key roll-call votes. Using the actual votes, we can, in turn, construct instrumental variables that correct for simultaneity. We find that the American electorate responds strongly to substantive representation. (1) Nearly all respondents have preferences over important bills before Congress. (2) Most constituents hold beliefs about their legislators’ roll-call votes that reflect both the legislators’ actual behavior and the parties’ policy reputations. (3) Constituents use those beliefs to hold their legislators accountable.