to the capitated managed care experiment that proved disastrous in the 1990s.  In both instances, providers receive a set annual payment to cover the costs of all care, and get to keep whatever they don't spend on patients.  The obvious winning strategy - from a business point of view - is to recruit relatively healthy patients, offering luxurious  care for the healthy and minimally ill, and subtle cues that those with expensive illness would be better off elsewhere.  Neither risk adjustment nor quality monitoring schemes are up to the task of blunting these incentives. An ACO can game risk adjustments by ferreting out additional diagnoses that may be clinically unimportant but would up its capitation payment, and make its outcomes look better as well.  The Dartmouth group has already shown that more expensive providers label their patients with more diagnoses in this way.  Quality monitoring efforts measure only a tiny slice of what's important in medicine.  Overarching measures of quality like death rates and family/community well being are either too rare to measure in a statistically reliable manner, too subtle to capture with current or foreseeable measurement strategies, or too biased by differences in the baseline health of enrollees.  Evidence from the UK shows that providers will improve on the aspects of care that are measured, but neglect those that are not, and it's far from clear that monitoring of quality measures has actually improved quality or can prevent abuses. In sum, the ACO strategy remains an untested theory for