1) Accountable care organizations (ACOs) are groups of healthcare providers that coordinate care for patients and are accountable for the quality and costs of that care. 2) The core principles of ACOs are that they are provider-led, have payments linked to quality and cost improvements, and use performance metrics to support care improvements. 3) Compared to earlier HMOs, ACOs focus not just on costs but also on quality of care and patient satisfaction, aiming to achieve the "Triple Aim" of better care, lower costs, and improved health.