The conventional rationale underlying postconviction habeas corpus in the Federal forum is that the individual's interest in freedom from unlawful detention warrants a second look at Federal claims already rejected by the State courts. The subject-matter jurisdiction of the habeas courts is explicitly limited to petitions from applicants who allege they are in 'custody' in violation of Federal Law. The courts, however, circumvent the 'custody' requirement if it threatens to obstruct effective Federal postconviction review to protect persons from recalcitrant State authorities. The proposed alternative explanation of habeas corpus is that it makes available a Federal forum in which to enforce Federal rights that may be unpopular with the States. This theory accounts for both the availability of the Federal forum to State criminal defendants with Federal claims and for the postponement of Federal adjudication until after the completion of State proceedings. The article concludes with a discussion of the implications of the proposed explanation for related areas of the law and current habeas doctrine. 281 footnotes.