The Medicare accountable care organization (ACO) programs rely on delivery system integration and provider risk sharing to lower spending while improving quality of care. McWilliams and coauthors examined health care spending and quality of care for 4.29 million Medicare beneficiaries served by various types of health care provider groups, investigating how size-related differences in spending and quality varied by 2 factors considered central to ACO performance: group primary care orientation and provider risk sharing. Compared with smaller groups, larger hospital-based groups had substantially higher total per-beneficiary spending, higher 30-day readmission rates, and similar performance on 4 of 5 process measures of quality, despite minimal differences in clinical case mix. In contrast, larger independent physician groups performed better than smaller groups on all process measures and exhibited significantly lower per-beneficiary spending in counties where risk sharing by these groups was more common. Among all groups sufficiently large to participate in ACO programs (hospital based or not), a strong primary care orientation was associated with lower spending, fewer readmissions, and better quality of diabetes care. In an Invited Commentary, Epstein outlines the progress of improving care through greater accountability.