We argue that a model of judicial behavior that accounts for dierences in justices' ability and ideology provides a fruitful alternative for the empirical analysis of judicial decision-making around the world, and illustrate this by focusing on the case of the UK.We show that the model explains the decisions of the Lords of Appeal remarkably well, and improves the t of a purely ideological model. We use our estimates to tackle previously unaddressed questions about the relative role of justices' preferences and ability in the Appellate Committee.