Political decisions have the potential to greatly impact our lives. Think of decisions in relation to abortion or climate change, for example. This makes political legitimacy an important normative concern. But what makes political decisions legitimate? Are they legitimate in virtue of having support from the citizens? Democratic conceptions of political legitimacy answer in the affirmative. Such conceptions rightly highlight that legitimate political decision-making must be sensitive to disagreements among the citizens. But what if democratic decisions fail to track what there is most reason to do? What if a democratically elected government fails to take measures necessary to protect its population from threats related to climate change? Peter argues that the legitimacy of political decisions doesn''t just depend on respect for the citizens'' will; and defends a novel hybrid conception of political legitimacy, called the Epistemic Accountability conception. According to this conception, political legitimacy also depends on how political decision-making responds to evidence for what there is most reason to do. The Grounds of Political Legitimacy starts with an overview of the main ways in which philosophers have thought about political legitimacy, and identifies the epistemic accountability conception as an overlooked alternative. It then develops the epistemic accountability conception of political legitimacy and discusses its implications for legitimate political decision-making. Considering the norms that should govern political debate, it examines the role of experts in politics, and probes the responsibilities of democratically elected political leaders and as well as of citizens.