Jurisdiction is the extent of a power. This gives rise to an important distinction. Jurisdictional errors take an authority outside the scope of its power. Nonjurisdictional errors do not, even if they render the ensuing exercise of power unlawful. Unwelcome baggage has made the task of arriving at a coherent account of jurisdiction needlessly difficult. That baggage is the common assumption that lack of jurisdiction is the sole basis for judicial review. This is wrong. Abandoning it makes space for an attractive account of when an error is jurisdictional. An error is jurisdictional if (i) it is an instance of a kind of error, in the sense of ground of review, which could arise at the outset of proceedings, but (ii) it does not pertain to a collateral matter, the purpose of which is tangential to the purpose of the inquiry. I shall call this the revised temporal view of jurisdiction. It accepts the possibility of judicial review on both jurisdictional and nonjurisdictional grounds. But the distinction has practical significance for at least three reasons. First, only acts with a jurisdictional flaw are nullities. Second, jurisdictional questions take analytical priority over nonjurisdictional ones. Third, post hoc explanations are only acceptable for jurisdictional issues.