42 C.F.R. § 425.600
(a) An ACO may elect to operate under one of the following tracks:
(4) BASIC track. For agreement periods beginning on July 1, 2019, and in subsequent years, an ACO in the BASIC track operates under either a one-sided model or a two-sided model (as described under § 425.605), either sharing savings only or sharing both savings and losses with the Medicare program, as specified in this paragraph (a)(4).
(i) Levels of the BASIC track's glide path—(A) Phase-in of levels of the risk and reward. Under the BASIC track's glide path, the level of risk and potential reward phases in over the course of the agreement period in the following order:
(1) Level A. The ACO operates under a one-sided model as described under § 425.605(d)(1)(i).
(2) Level B. The ACO operates under a one-sided model as described under § 425.605(d)(1)(ii).
(3) Level C. The ACO operates under a two-sided model as described under § 425.605(d)(1)(iii).
(4) Level D. The ACO operates under a two-sided model as described under § 425.605(d)(1)(iv).
(5) Level E. The ACO operates under a two-sided model as described under § 425.605(d)(1)(v).
(B) Glide path progression for agreement periods beginning on or after July 1, 2019 and before January 1, 2024. (1) Experience in Track 1. (i) Except for an ACO that previously participated in Track 1 under paragraph (a)(1) of this section or a new ACO identified as a re-entering ACO because more than 50 percent of its ACO participants have recent prior experience in a Track 1 ACO, an ACO eligible to enter the BASIC track's glide path as determined under paragraphs (d)(1)(i) and (d)(2)(i) of this section may elect to enter its agreement period at any of the levels of risk and potential reward available under paragraphs (a)(4)(i)(A)(1) through (5) of this section.
(ii) An ACO that previously participated in Track 1 under paragraph (a)(1) of this section or a new ACO identified as a re-entering ACO because more than 50 percent of its ACO participants have recent prior experience in a Track 1 ACO may elect to enter its agreement period at any of the levels of risk and potential reward available under paragraphs (a)(4)(i)(A)(2) through (5) of this section.
(2) Automatic advancement. Unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i), the ACO is automatically advanced to the next level of the BASIC track's glide path at the start of each subsequent performance year of the agreement period, if a higher level of risk and potential reward is available under the BASIC track.
(i) Exception for ACO entering the BASIC track's glide path for an agreement period beginning on July 1, 2019. The automatic advancement does not apply at the start of the second performance year for an ACO entering the BASIC track's glide path for an agreement period beginning on July 1, 2019. For performance year 2020, the ACO remains in the same level of the BASIC track's glide path that it entered for the July 1, 2019 through December 31, 2019 performance year, unless the ACO chooses to advance more quickly in accordance with § 425.226(a)(2)(i). The ACO is automatically advanced to the next level of the BASIC track's glide path at the start of performance year 2021 and all subsequent performance years of the agreement period.
(ii) Exception for new legal entity identified as a low revenue ACO. An exception is available for a low revenue ACO that is a new legal entity and is not identified as a re-entering ACO that enters the BASIC track's glide path at Level A under paragraph (a)(4)(i)(A)(1) of this section, and is automatically advanced to Level B under paragraph (a)(4)(i)(A)(2) of this section for performance year 2 (or performance 3 in the case of ACOs entering an agreement period beginning on July 1, 2019). Prior to the automatic advancement of the ACO to Level C under paragraph (a)(4)(i)(A)(3) of this section, the ACO may elect to remain in Level B under paragraph (a)(4)(i)(A)(2) of this section for performance year 3 (performance year 4 in the case of ACOs entering an agreement period beginning on July 1, 2019). In the case of an ACO that elects to remain in Level B for an additional performance year pursuant to the second sentence of paragraph (a)(4)(i)(B)(2)(ii) of this section, and except as provided in paragraph (a)(4)(i)(B)(2)(vi) of this section, the ACO is automatically advanced to Level E under paragraph (a)(4)(i)(A)(5) of this section at the start of performance year 4 (or performance year 5 in the case of ACOs entering an agreement period beginning on July 1, 2019).
(iii) Exception for ACOs participating in the BASIC track's glide path that elect to maintain their participation level for performance year 2021. Prior to the automatic advancement for performance year 2021, an ACO that is participating in the BASIC track's glide path for performance year 2020 may elect to remain in the same level of the BASIC track's glide path that it entered for the 2020 performance year, for performance year 2021. For performance year 2022, the ACO is automatically advanced to the level of the BASIC track's glide path to which the ACO would have automatically advanced absent the election to maintain its participation level for performance year 2021, unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i). A voluntary election by an ACO under this paragraph must be made in the form and manner and by a deadline established by CMS.
(iv) Exception for ACOs participating in the BASIC track's glide path that elect to maintain their participation level for performance year 2022. Prior to the automatic advancement for performance year 2022, an ACO that is participating in the BASIC track's glide path for performance year 2021 may elect to remain in the same level of the BASIC track's glide path in which it participated during the 2021 performance year, for performance year 2022. Except as provided in paragraph (a)(4)(i)(B)(2)(vi) of this section, for performance year 2023, the ACO is automatically advanced to the level of the BASIC track's glide path to which the ACO would have automatically advanced absent the election to maintain its participation level for performance year 2022 and, if applicable, the election to maintain its participation level for performance year 2021 under paragraph (a)(4)(i)(B)(2)(iii) of this section, unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i). A voluntary election by an ACO under this paragraph must be made in the form and manner and by a deadline established by CMS.
(v) Prior to entering performance-based risk, an ACO must meet all requirements to participate under performance-based risk, including establishing an adequate repayment mechanism as specified under § 425.204(f) and selecting a MSR/MLR from the options specified under § 425.605(b).
(vi) For performance year 2023, an ACO in Level A under paragraph (a)(4)(i)(A)(1) of this section or in Level B under paragraph (a)(4)(i)(A)(2) of this section may elect to remain in the same level of the BASIC track's glide path in which it participated during performance year 2022, for the remainder of the agreement period, unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i). If the ACO does not elect to remain under Level A or Level B, for performance year 2023, the ACO is automatically advanced to the next level of the BASIC track's glide path to which the ACO would have automatically advanced absent any election to maintain its participation level for performance year 2022 under paragraph (a)(4)(i)(B)(2)(iv) of this section and, if applicable, the election to maintain its participation level for performance year 2021 under paragraph (a)(4)(i)(B)(2)(iii) of this section, unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i). A voluntary election by an ACO under this paragraph must be made in the form and manner and by a deadline established by CMS.
(vii) For performance year 2024, an ACO with an agreement period beginning January 1, 2023 in Level A under paragraph (a)(4)(i)(A)(1) of this section or in Level B under paragraph (a)(4)(i)(A)(2) of this section may elect to remain in the same level of the BASIC track's glide path in which it participated during performance year 2023, for the remainder of the agreement period, unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i). If the ACO does not elect to remain under Level A or Level B, for performance year 2024, the ACO is automatically advanced to the next level of the BASIC track's glide path, unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i). A voluntary election by an ACO under this paragraph must be made in the form and manner and by a deadline established by CMS.
(3) If the ACO fails to meet the requirements to participate under performance-based risk under paragraph (a)(4)(i)(B)(2)(v) of this section, the agreement is terminated.
(4) If, in accordance with § 425.226(a)(2)(i), the ACO elects to transition to a higher level of risk and reward available under paragraphs (a)(4)(i)(A)(3) through (5) of this section, then the automatic transition to levels of higher risk and reward specified in paragraph (a)(4)(i)(B)(2) of this section applies to all subsequent performance years of the agreement period.
(C) Glide path progression for agreement periods beginning on or after January 1, 2024. (1) Level of glide path entry. An ACO eligible to enter the BASIC track's glide path as determined under paragraphs (g)(1) or (h)(1) of this section may elect to enter its agreement period at any of the levels of risk and potential reward available under paragraphs (a)(4)(i)(A)(1) through (5) of this section.
(2) Automatic advancement. An ACO is automatically advanced to the next level of the BASIC track's glide path at the start of each subsequent performance year of the agreement period, if a higher level of risk and potential reward is available under the BASIC track, except as follows:
(i) The ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i).
(ii) The ACO elects to maintain its level of participation as provided in paragraph (a)(4)(i)(C)(3) of this section.
(iii) The ACO is automatically advanced to Level E pursuant to paragraph paragraph (i)(2)(i) of this section.
(3) Election to remain under a one-sided model. An eligible ACO that enters the BASIC track's glide path at Level A under paragraph (a)(4)(i)(A)(1) of this section and is currently at Level A may elect to remain in Level A under paragraph (a)(4)(i)(A)(1) of this section for all subsequent performance years of the agreement period.
(i) To be eligible to participate under Level A of the BASIC track as described in this paragraph, the ACO must meet the following requirements: the ACO is participating in its first agreement period under the BASIC track under paragraph (a)(4) of this section, and is not participating in an agreement period under the BASIC track as a renewing ACO (as defined at § 425.20) or a re-entering ACO (as defined in § 425.20) that previously participated in the BASIC track's glide path under paragraph (a)(4) of this section; and the ACO is inexperienced with performance-based risk Medicare ACO initiatives (as defined in § 425.20).
(ii) A voluntary election by an ACO under this paragraph (a)(4)(i)(C)(3) must be made in the form and manner and by a deadline established by CMS.
(iii) The ACO's election to remain in Level A applies for the entirety of the agreement period, unless the ACO elects to transition to a higher level of risk and potential reward within the BASIC track's glide path as provided in § 425.226(a)(2)(i).
(4) Prior to entering performance-based risk, an ACO must meet all requirements to participate under performance-based risk, including establishing an adequate repayment mechanism as specified under § 425.204(f) and selecting a MSR/MLR from the options specified under § 425.605(b).
(5) If the ACO fails to meet the requirements to participate under performance-based risk under paragraph (a)(4)(i)(C)(4) of this section, the agreement is terminated.
(6) If, in accordance with § 425.226(a)(2)(i), the ACO elects to transition to a higher level of risk and reward available under paragraphs (a)(4)(i)(A)(3) through (5) of this section, then the automatic transition to levels of higher risk and reward specified in paragraph (a)(4)(i)(C)(2) of this section applies to all subsequent performance years of the agreement period.
(b) For agreement periods beginning before July 1, 2019, ACOs may operate under the one-sided model for a maximum of 2 agreement periods. An ACO may not operate under the one-sided model for a second agreement period unless the—
(d) For agreement periods beginning on or after July 1, 2019, and before January 1, 2024, CMS determines an ACO's eligibility for the Shared Savings Program participation options specified in paragraph (a) of this section as follows:
(1) If an ACO is identified as a high revenue ACO, the ACO is eligible for the participation options indicated in paragraph (a) of this section as follows:
(ii) If the ACO is determined to be experienced with performance-based risk Medicare ACO initiatives:
(2) If an ACO is identified as a low revenue ACO, the ACO is eligible for the participation options indicated in paragraph (a) of this section as follows:
(3) Low revenue ACOs may participate under the BASIC track for a maximum of two agreement periods. A low revenue ACO may only participate in the BASIC track for a second agreement period if it satisfies either of the following:
(e) For performance years beginning on or after July 1, 2019 and before January 1, 2024, CMS monitors low revenue ACOs identified as experienced with performance-based risk Medicare ACO initiatives, during an agreement period in the BASIC track, for changes in the revenue of ACO participants that would cause the ACO to be considered a high revenue ACO and ineligible for participation in the BASIC track. If the ACO meets the definition of a high revenue ACO (as specified in § 425.20)—
(f) For agreement periods beginning on July 1, 2019, and in subsequent years, CMS determines the agreement period an ACO is entering for purposes of applying program requirements that phase-in over multiple agreement periods, as follows:
(2) A re-entering ACO is considered to be entering a new agreement period in the Shared Savings Program as follows—
(iii) A new ACO identified as a re-entering ACO enters the program in an agreement period that is determined based on the prior participation of the ACO in which the majority of the new ACO's participants were participating.
(4) For purposes of this paragraph (f), program requirements that phase in over multiple agreement periods are as follows:
(g) For agreement periods beginning on or after January 1, 2024 and before January 1, 2027, CMS determines an ACO's eligibility for the Shared Savings Program participation options specified in paragraph (a) of this section as follows:
(1) If an ACO is determined to be inexperienced with performance-based risk Medicare ACO initiatives, the ACO may enter either the BASIC track's glide path at any of the levels of risk and potential reward under paragraphs (a)(4)(i)(A)(1) through (5) of this section, or the ENHANCED track under paragraph (a)(3) of this section.
(ii) An ACO that enters an agreement under the BASIC track's glide path at either Level A under paragraph (a)(4)(i)(A)(1) of this section or Level B under paragraph (a)(4)(i)(A)(2) of this section is deemed to have completed one agreement under the BASIC track's glide path and is only eligible to enter a second agreement under the BASIC track's glide path if the ACO continues to meet the definition of inexperienced with performance-based risk Medicare ACO initiatives and satisfies either of the following:
(h) For agreement periods beginning on or after January 1, 2027, CMS determines an ACO's eligibility for the Shared Savings Program participation options specified in paragraph (a) of this section as follows:
(ii) An ACO that enters an agreement period under the BASIC track's glide path at any of the levels of risk and potential reward available under paragraphs (a)(4)(i)(A)(1) through (5) of this section is deemed to have completed one agreement period under the BASIC track's glide path. For the purpose of determining the ACO's prior participation in the BASIC track's glide path, CMS considers whether the ACO satisfies either of the following:
(3) If an ACO is determined to have fewer than 5,000 assigned beneficiaries in either the first benchmark year, the second benchmark year, or both, in accordance with § 425.110(a)(3), the ACO may only enter the BASIC track. The ACO may enter a level of risk and potential reward under the BASIC track in accordance with the requirements of this paragraph, as follows:
(2) If the ACO meets the definition of experienced with performance-based risk Medicare ACO initiatives (under § 425.20)—
[76 FR 67973, Nov. 2, 2011, as amended at 80 FR 32841, June 9, 2015; 83 FR 68069, Dec. 31, 2018; 85 FR 27625, May 8, 2020; 85 FR 85041, Dec. 28, 2020; 86 FR 45521, Aug. 13, 2021; 87 FR 70235, Nov. 18, 2022; 88 FR 79547, Nov. 16, 2023; 90 FR 50017, Nov. 5, 2025]