---
kind: "section"
citation: "10 U.S.C. § 4321"
title: "10"
title_heading: "Armed Forces"
number: "4321"
heading: "Development of major defense acquisition programs: sustainment of system to be replaced"
release: "119-102"
date: "2026-07-12"
url: "https://uscodex.org/usc/10/4321"
units:
  - "Subtitle A — General Military Law"
  - "Part V — Acquisition"
  - "Subpart F — Major Systems, Major Defense Acquisition Programs, and Weapon Systems Development"
  - "Chapter 323 — Life-Cycle and Sustainment"
---

# §4321. Development of major defense acquisition programs: sustainment of system to be replaced

- (a) **Requirement for Sustaining Existing Forces.—**
  - (1) The [Secretary](/usc/10/4951.md?p=3) of Defense shall require that, whenever a new [major defense acquisition program](/usc/10/4201.md?p=a) begins development, the defense acquisition authority responsible for that [program](/usc/10/2101.md?p=1) shall develop a plan (to be known as a “sustainment plan”) for the existing system that the system under development is intended to replace. Any such sustainment plan shall provide for an appropriate level of budgeting for sustaining the existing system until the replacement system to be developed under the [major defense acquisition program](/usc/10/4201.md?p=a) is fielded and assumes the majority of responsibility for the mission of the existing system. This section does not apply to a major defense acquisition that reaches initial operational capability before October 1, 2008.
  - (2) In this section, the term “defense acquisition authority” means the [Secretary](/usc/10/4951.md?p=3) of a [military](/usc/10/801.md?p=8) [department](/usc/10/101.md?p=a-6) or the commander of the [United States](/usc/10/101.md?p=a-1) Special Operations Command.
- (b) **Sustainment Plan.—** The [Secretary](/usc/10/4951.md?p=3) of Defense shall require that each sustainment plan under this section include, at a minimum, the following:
  - (1) The milestone schedule for the development of the [major defense acquisition program](/usc/10/4201.md?p=a), including the scheduled dates for low-[rate](/usc/10/101.md?p=b-9) initial production, initial operational capability, full-[rate](/usc/10/101.md?p=b-9) production, and full operational capability and the date as of when the replacement system is scheduled to assume the majority of responsibility for the mission of the existing system.
  - (2) An analysis of the existing system to assess the following:
    - (A) Anticipated funding levels necessary to—
      - (i) ensure acceptable reliability and availability [rates](/usc/10/101.md?p=b-9) for the existing system; and
      - (ii) maintain mission capability of the existing system against the relevant threats.
    - (B) The extent to which it is necessary and appropriate to—
      - (i) transfer mature technologies from the new system or other systems to enhance the mission capability of the existing system against relevant threats; and
      - (ii) provide interoperability with the new system during the period from initial fielding until the new system assumes the majority of responsibility for the mission of the existing system.
- (c) **Exceptions.—** [Subsection (a)](#a) shall not apply to a [major defense acquisition program](/usc/10/4201.md?p=a) if the [Secretary](/usc/10/4951.md?p=3) of Defense determines that—
  - (1) the existing system is no longer relevant to the mission;
  - (2) the mission has been eliminated;
  - (3) the mission has been consolidated with another mission in such a manner that another existing system can adequately meet the mission requirements; or
  - (4) the duration of time until the new system assumes the majority of responsibility for the existing system’s mission is sufficiently short so that mission availability, capability, interoperability, and force protection requirements are maintained.
- (d) **Waiver.—** The [Secretary](/usc/10/4951.md?p=3) of Defense may waive the applicability of [subsection (a)](#a) to a [major defense acquisition program](/usc/10/4201.md?p=a) if the [Secretary](/usc/10/4951.md?p=3) determines that, but for such a waiver, the [Department](/usc/10/101.md?p=a-6) would be unable to meet [national security](/usc/10/801.md?p=16) objectives. Whenever the [Secretary](/usc/10/4951.md?p=3) makes such a determination and authorizes such a waiver, the [Secretary](/usc/10/4951.md?p=3) shall submit notice of such waiver and of the [Secretary](/usc/10/4951.md?p=3)’s determination and the reasons therefor in writing to the [congressional defense committees](/usc/10/101.md?p=a-16).

## Source credit

(Added Pub. L. 108–375, div. A, title VIII, § 805(a)(1), Oct. 28, 2004, 118 Stat. 2008, § 2437; renumbered § 4321, Pub. L. 116–283, div. A, title XVIII, § 1848(b), Jan. 1, 2021, 134 Stat. 4258.)

## Notes

### Editorial Notes

### Prior Provisions

A prior section 4321 was renumbered section 7421 of this title.

### Amendments

2021—Pub. L. 116–283 renumbered section 2437 of this title as this section.

### Statutory Notes and Related Subsidiaries

### Effective Date of 2021 Amendment

Amendment by Pub. L. 116–283 effective Jan. 1, 2022, with additional provisions for delayed implementation and applicability of existing law, see section 1801(d) of Pub. L. 116–283, set out as a note preceding section 3001 of this title.

### Effective Date

Pub. L. 108–375, div. A, title VIII, § 805(b), Oct. 28, 2004, 118 Stat. 2009, provided that: “Section 2437 of title 10, United States Code [now 10 U.S.C. 4321], as added by subsection (a), shall apply with respect to a major defense acquisition program for a system that is under development as of the date of the enactment of this Act [Oct. 28, 2004] and is not expected to reach initial operational capability before October 1, 2008. The Secretary of Defense shall require that a sustainment plan under that section be developed not later than one year after the date of the enactment of this Act for the existing system that the system under development is intended to replace.”