Because the brain is vastly more complex and more dynamic than other organs, and many different processes and experiences dynamically interact to create the unique pattern and flavor of symptoms an individual may express, its operations are only slowly yielding to understanding. That enormous heterogeneity makes precision psychiatry more of an aspiration than a reality. Nevertheless, precision psychiatry aims to identify and understand the many neurobiological mechanisms that underlie the symptoms of psychiatric disorders to apply treatments and prevention strategies that target them.