It is difficult to know exactly how many coverage denials there are because the American health care system is notoriously fragmented. This chapter introduces a novel survey of 1,340 US adults, finding that 36 percent of respondents have been denied at least once, and typically experience multiple denials. Those in private health insurance experience denials to the greatest degree, which is consistent with private insurers’ cost containment objectives. Health care can be denied for any number of reasons, such as the lack of medical necessity, the absence of a required prior authorization, or an assessment that the care is experimental or investigational. Combining survey and interview evidence, the chapter highlights the destabilizing impact that denials – whether pre-treatment (as with prior authorization) or post-treatment (as with emergency care) can have on patients, whose trust in the health insurance system can be shaken. While prior authorization and other coverage denials can constitute important ways to guard against overtreatment in the American health care system, this chapter presents new evidence that managed care tools may overcorrect, instead denying appropriate care.