With the proliferation of "accountable care" initiatives in both the private sector and the Medicare program, there will soon be several hundred health care organizations that have taken responsibility for a defined population of patients and are receiving financial incentives for improving care and slowing cost growth. In a blog post, Elliott Fisher, M.D., of The Dartmouth Institute for Health Policy and Clinical Practice and Stephen Shortell, Ph.D., of the University of California, Berkeley, School of Public Health highlight four areas that must be carefully examined to ensure accountable care organizations meet their promise.