The U.S. mental health workforce is varied and flexible. The strong growth in supply of nonphysician mental health professionals, ranging from psychologists to "midlevel" professionals like social workers and nurse specialists, helps to offset the dwindling numbers of medical graduates entering the field of psychiatry. Primary care physicians often see patients who have some form of mental illness, which they are not always trained to recognize and treat. The data on the supply of several specialists--psychiatrists, clinical psychologists, and clinical social workers--indicate that the distribution of mental health professionals varies widely by state. The composition, supply, and distribution of workers in this field also affect the care of vulnerable populations. Broader policy questions, including the lack of parity between mental and physical health insurance coverage and barriers to entry by nonphysician professions, may limit the cost-effective expansion of this diverse and dynamic workforce.