As a young attending physician at a Connecticut medical center 35 years ago, Sharon Inouye was shocked by the disturbing changes she saw in many older patients. They would arrive at the hospital clear-headed and focused but soon became confused and disoriented—for no obvious or consistent reason. Some developed delusions and thrashing agitation; others seemed sedated and out of it. “I asked other physicians about it, and they were dismissive,” she recalls. This muddled state known as delirium “was taken as an expected thing” for older patients, but Inouye found it to be both unacceptable and deeply interesting. Now a geriatrician and professor at Harvard Medical School, she is one of the world’s leading investigators of delirium, the toll it can take and how to prevent it.