At least 5.6 million to 8 million-nearly one in five-older adults in America have one or more mental health and substance use conditions, which present unique challenges for their care. With the number of adults age 65 and older projected to soar from 40.3 million in 2010 to 72.1 million by 2030, the aging of America holds profound consequences for the nation. For decades, policymakers have been warned that the nation's health care workforce is ill-equipped to care for a rapidly growing and increasingly diverse population. In the specific disciplines of mental health and substance use, there have been similar warnings about serious workforce shortages, insufficient workforce diversity, and lack of basic competence and core knowledge in key areas. Following its 2008 report highlighting the urgency of expanding and strengthening the geriatric health care workforce, the IOM was asked by the Department of Health and Human Services to undertake a complementary study on the geriatric mental health and substance use workforce. The Mental Health and Substance Use Workforce for Older Adults: In Whose Hands? assesses the needs of this population and the workforce that serves it. The breadth and magnitude of inadequate workforce training and personnel shortages have grown to such proportions, says the committee, that no single approach, nor a few isolated changes in disparate federal agencies or programs, can adequately address the issue. Overcoming these challenges will require focused and coordinated action by all.