Psychiatry has long sought to identify and validate biomarkers of brain and mental health, and biomarkers are thought to be necessary for the advancement of precision psychiatry. For millions of people who currently undergo pharmacotherapy for depression, for example, treatment usually involves a trial-and-error process with different drugs, several rounds of therapeutic failure, unpleasant or intolerable side effects, and many months before there is any relief, if at all. Researchers are now actively testing whether biomarkers of nerve-connectivity patterns in various brain circuits could help identify which people are most likely to respond to various kinds of antidepressants. Genomic testing can also yield information about who is more likely to respond to classes of drugs or specific drugs.