The Affordable Care Act (ACA) reshaped the U.S. health insurance system by setting rules that prevent insurers from denying coverage based on health history, requiring plans to cover a broad set of medical services, and creating financial assistance so more people can afford insurance. Since its passage in 2010, the law has expanded coverage to tens of millions of Americans through a combination of consumer protections, subsidized marketplace plans, and Medicaid expansion. As of the 2025 open enrollment period, 24.3 million people selected or were automatically re-enrolled in marketplace coverage alone.
Protection for Pre-Existing Conditions
Before the ACA, insurers could refuse to sell you a policy, charge you significantly more, or exclude specific treatments if you had a health problem before applying. The law eliminated all of that. Insurance companies cannot deny coverage, charge higher premiums, or limit benefits because of a pre-existing condition like asthma, diabetes, cancer, or pregnancy. Once you have a plan, the insurer also cannot refuse to cover treatment for that condition.
The one exception involves “grandfathered” plans, meaning plans that existed before the ACA was signed and haven’t made significant changes since. These plans don’t have to follow the pre-existing condition rules, though they’re increasingly rare.
Ten Categories of Required Coverage
The ACA established a minimum standard for what health insurance actually has to include. All individual and small group market plans must cover services in ten categories:
- Outpatient care (doctor visits and services that don’t require hospital admission)
- Emergency services
- Hospitalization
- Maternity and newborn care
- Mental health and substance use disorder treatment
- Prescription drugs
- Rehabilitative services and devices
- Laboratory services
- Preventive and wellness services, including chronic disease management
- Pediatric services (including dental and vision care for children)
Before the ACA, many individual plans didn’t cover maternity care, mental health treatment, or prescription drugs at all. The law made these non-negotiable.
Free Preventive Care
Most health plans must cover a set of preventive services at no cost to you, meaning no copay, no coinsurance, and no need to meet your deductible first. This applies when you use an in-network provider and includes services like immunizations, screening tests, and wellness exams. Coverage is organized by group: all adults, women, and children each have specific services designated as zero-cost. The practical effect is that routine screenings for things like high blood pressure, cholesterol, diabetes, and certain cancers are free, removing the financial barrier to catching problems early.
Financial Help for Buying Insurance
The ACA created health insurance marketplaces (often called “exchanges”) where individuals and families can shop for plans and, in many cases, get financial help paying premiums. The main form of assistance is the premium tax credit, which lowers your monthly cost based on household income. To qualify, your income generally needs to fall between 100% and 400% of the federal poverty level for your family size.
In 2021, Congress temporarily expanded these subsidies through the American Rescue Plan Act, removing the 400% income cap and making credits more generous across the board. The Inflation Reduction Act extended those enhanced credits through the end of 2025. If they expire without renewal, marketplace enrollees are estimated to see premium payments jump by an average of 114%, roughly $1,016 more per year. Whether these enhanced subsidies continue depends on future legislation.
Medicaid Expansion
The ACA originally intended to expand Medicaid nationwide to cover all adults with household incomes below 138% of the federal poverty level. A 2012 Supreme Court ruling made expansion optional for states, and not all states have adopted it. In states that did expand, Medicaid covers a much broader group of low-income adults who previously wouldn’t have qualified, particularly adults without children. In states that haven’t expanded, many low-income adults fall into a “coverage gap” where they earn too much for traditional Medicaid but too little to qualify for marketplace subsidies.
Staying on a Parent’s Plan Until 26
If a parent’s health insurance plan covers dependents, their children can stay on that plan until they turn 26. This applies to job-based plans regardless of whether the young adult is married, has children, is in school, lives at home, or has access to their own employer coverage. For marketplace plans, coverage lasts through December 31 of the year you turn 26. Some states allow dependents to stay on even longer. This provision has been one of the law’s most widely used features, covering millions of young adults who might otherwise go uninsured during their early career years.
Requirements for Employers
Businesses with 50 or more full-time employees (called “applicable large employers”) are required to offer health insurance to their workers or face a financial penalty. Full-time means averaging at least 30 hours per week, or 130 hours per month. The employer calculates its workforce size by averaging full-time and full-time equivalent employees over the prior calendar year. Smaller businesses are exempt from this mandate, though many still choose to offer coverage voluntarily, and some qualify for tax credits to help cover the cost.
The 80/20 Rule
The ACA requires insurance companies to spend at least 80% of the premium dollars they collect on actual medical care and quality improvement. The remaining 20% can go toward administrative costs, overhead, and marketing. For insurers selling to large groups (typically more than 50 employees), the threshold is even higher at 85%. If an insurer doesn’t meet these ratios, it has to issue rebates to policyholders. This rule, formally called the medical loss ratio requirement, was designed to prevent insurers from collecting premiums and funneling most of the money into profits and administrative overhead rather than patient care.