Accountable Care Organizations: Structure and Role in US Healthcare

Accountable Care Organizations — ACOs — are one of the more consequential structural experiments in American healthcare financing, linking provider payment directly to the quality and cost of care delivered to a defined patient population. Born from the Affordable Care Act and formalized through Medicare's Shared Savings Program, they represent a deliberate departure from fee-for-service medicine toward a model where keeping people healthy, not simply treating illness, has financial value. This page covers how ACOs are structured, how the incentive mechanics actually work, and where they fit within the broader landscape of US healthcare policy.

Definition and scope

An Accountable Care Organization is a network of doctors, hospitals, and other healthcare providers that voluntarily agrees to coordinate care for a defined group of patients and be held accountable — financially and by quality metrics — for the outcomes and costs of that care. The Centers for Medicare & Medicaid Services (CMS) defines ACOs under the Medicare Shared Savings Program (MSSP) as groups of providers who "work together to give coordinated, high quality care" to Medicare fee-for-service beneficiaries (CMS MSSP Overview).

The scale is not trivial. As of 2023, CMS reported approximately 480 ACOs participating in the MSSP, covering roughly 10.8 million assigned Medicare beneficiaries (CMS MSSP Fast Facts, 2023). Private-payer ACO arrangements add millions more across commercial insurance networks, though those figures vary by insurer and are not centrally tracked.

ACOs operate at the intersection of primary care, specialty care, and chronic disease management — which is precisely why they tend to work best when those three functions are tightly connected rather than siloed.

How it works

The fundamental mechanic is shared savings — and, in some models, shared risk. CMS sets a benchmark spending target for each ACO based on the historical costs of its attributed patient population. If the ACO delivers care below that benchmark while meeting a defined set of quality thresholds, it keeps a portion of the savings. If it exceeds the benchmark, consequences depend on the track the ACO has chosen.

The MSSP currently offers two broad participation tracks:

1. BASIC track — Entry-level participation with lower financial risk. ACOs in early levels of the BASIC track are eligible for shared savings but are not liable for shared losses. This is the typical starting point for newly forming ACOs.
2. ENHANCED track — Higher potential savings (up to 75% of generated savings) in exchange for accepting downside risk, meaning the ACO owes money back to CMS if costs exceed the benchmark. This track mirrors two-sided risk arrangements common in commercial ACO contracts.

Quality measurement runs through roughly 30 individual measures under the ACO REACH and MSSP programs, covering domains like preventive care, chronic disease management, patient experience, and avoidable hospitalizations. An ACO can generate savings on paper but forfeit its shared savings payment by failing quality thresholds — a feature designed to prevent cost-cutting at the expense of patient welfare.

Attribution of patients to an ACO typically happens automatically based on where a beneficiary receives the plurality of their primary care services. Patients are not formally enrolled and may not know they belong to an ACO's attributed population.

Common scenarios

The ACO model shows up in a few recurring real-world configurations:

• Physician-led ACOs — A large primary care group or independent practice association anchors the organization, sometimes contracting with hospitals rather than including them as equity partners. These tend to prioritize preventive care and screenings and care coordination over inpatient management.

• Hospital-led ACOs — A health system forms an ACO using its employed physician network and affiliated facilities. These carry the inherent tension of a hospital being incentivized to reduce admissions that are otherwise a revenue source under fee-for-service.

• Rural and community health ACOs — Smaller networks built around community health centers or critical access hospitals, often serving populations with higher rates of uninsured and underinsured patients. CMS has created specific MSSP accommodations for low-revenue ACOs recognizing that rural organizations face structurally different cost profiles.

In all configurations, ACOs invest heavily in care coordinators, data analytics platforms, and transitional care programs — particularly around hospital discharge, which is one of the highest-risk moments for chronic disease patients.

Decision boundaries

Not every provider arrangement qualifies as an ACO, and the differences matter. A clinically integrated network (CIN) may coordinate care and share data without accepting financial risk — making it functionally different from an ACO even if the organizational chart looks similar. A narrow-network health plan is a payer construct, not a provider accountability structure.

A few distinctions worth holding clearly:

• ACO vs. HMO — A health maintenance organization controls access through gatekeeping and prior authorization. An ACO coordinates care but does not restrict access; patients can see any Medicare-participating provider, even outside the ACO's network.
• ACO vs. bundled payment — Bundled payments attach a single payment to a specific episode of care (a hip replacement, for example). ACOs apply to the full continuum of a patient's care over a performance year, regardless of what conditions arise.
• MSSP ACO vs. ACO REACH — ACO Realizing Equity, Access, and Community Health (REACH), launched in 2023, targets higher-risk populations and introduces a capitated payment structure with mandatory health equity data collection, moving further from pure fee-for-service than the standard MSSP framework.

Understanding where ACOs fit within the full architecture of healthcare coverage options requires recognizing that they are fundamentally a payment and accountability reform — one that reshapes how existing coverage is administered rather than replacing the insurance structures underneath it. The question of whether the model actually bends the cost curve is still being answered, with CMS evaluation data suggesting that mature, experienced ACOs generate more consistent savings than newly formed ones, pointing less to a structural flaw and more to a learning curve.