Cite as: 531 U. S. 230 (2001)
Opinion of the Court
Lopez's position would confine the BOP's discretion under § 3621(e)(2)(B) to consideration of factors of the kind the Bureau already may consider in granting credit for "satis-factory behavior." See 18 U. S. C. § 3624(b)(1) ("a prisoner [serving a term of more than one year and less than life] may receive credit toward the service of the prisoner's sentence . . . subject to determination by the Bureau of Prisons that, during that year, the prisoner has displayed exemplary compliance with such institutional disciplinary regulations").
We turn now to the Bureau's reading of the statutory text, which instructs that the agency "may" reduce the sentence of a nonviolent offender who has successfully completed a drug treatment program. Congress' use of the permissive "may" in § 3621(e)(2)(B) contrasts with the legislators' use of a mandatory "shall" in the very same section. Elsewhere in § 3621, Congress used "shall" to impose discretionless obligations, including the obligation to provide drug treatment when funds are available. See 18 U. S. C. § 3621(e)(1) ("Bureau of Prisons shall, subject to the availability of appropriations, provide residential substance abuse treatment (and make arrangements for appropriate aftercare)"); see also, e. g., § 3621(b) ("The Bureau shall designate the place of the prisoner's imprisonment. . . . In designating the place of imprisonment or making transfers under this subsection, there shall be no favoritism given to prisoners of high social or economic status."). Sensibly read, the grant of discretion in § 3621(e)(2)(B) to decide whether to reduce a sentence parallels the grant of discretion in § 3621(e)(2)(A) to retain a prisoner who successfully completes drug treatment "under such [custodial] conditions as the Bureau deems appropriate." § 3621(e)(2)(A). When an eligible prisoner successfully completes drug treatment, the Bureau thus has the authority, but not the duty, both to alter the prisoner's conditions of confinement and to reduce his term of imprisonment.
The constraints Lopez urges—requiring the BOP to make individualized determinations based only on postconviction
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