In Zadvydas v. Davis, decided in June 2001, a narrow majority of the Supreme Court held that deportable aliens who had spent several years in this country may not be detained indefinitely if their country of nationality refuses their return, even if they have serious criminal records. The ruling technically rests on statutory construction but was heavily influenced, under the doctrine of constitutional doubt, by the Court's lengthy discussion of due process rights in this setting. At the same time, the Court basically reaffirmed the 1953 Mezei decision, which allowed indefinite detention of an excludable alien refused admission at the border. This paper explores several possible explanations for the Court's due process doctrine, and then draws on that exploration to suggest that graduated application of constitutional protection to aliens makes sense, but not according to the simple two-level model that much of the Zadvydas discussion seems to support. It identifies six different and sharply differentiated categories along the citizen/alien spectrum that are deeply rooted in American law and practice: citizen, lawful permanent resident alien, admitted nonimmigrant (admitted for a temporary stay), entrant without inspection, parolee, and applicant for admission at the border. Constitutional protection is logically graduated according to this hierarchy, and the division between lawful permanent residents and other aliens should be accorded greater significance than it has generally received. Also, those on the lowest levels should not necessarily be considered to have no constitutional protection. The article then applies the analysis to several specific constitutional questions: a closer look at indefinite detention of deportable and excludable aliens, whether some lawfully resident aliens should be considered immune to deportation, and under what circumstances the use of secret evidence should be judged constitutional in immigration cases.