Accountable Care Organizations (ACOs) are networks of providers that assume risk for the quality and total cost of the care they deliver. Public policymakers and private insurers hope that ACOs will achieve the elusive “triple aim” of improving quality of care, improving population health, and reducing costs. The model is still evolving, but the premise is that ACOs will accomplish these aims by coordinating care, managing chronic disease, and aligning financial incentives for hospitals and physicians. If this sounds familiar, it may be because the integrated care networks of the 1990s tried some of the same things, and mostly failed in their attempts. This Issue Brief summarizes the similarities and differences between the new ACOs and the integrated delivery networks of the 1990s, and presents the authors’ analysis of the likely success of these new organizations in affecting the costs and quality of health care.