This chapter aims to provide an overview of the burgeoning field of psychoneuroimmunology as it relates to psychiatric and neuropsychiatric disorders. It is a relatively young field, having come of age only recently, but the progress that has been made just in the past three decades has exceeded all expectations. It is fair to say that the field has opened up new horizons in our understanding of the complex interrelationships between the immune and nervous systems or as is otherwise referred to as the brain-immune interaction. Hitherto unknown biochemical pathways have been identified, and their complex interactions with neurotransmitters and immune mediators present opportunities for innovative research and identification of new targets for drug development. Biomarkers are being established that hold great promise for more precise diagnostic classification of psychiatric disorders but also understanding of the high comorbidity between specific psychiatric disease entities and a host of medical and neurological diseases. At the same time, immune biomarkers, neurotrophins, and antibodies enable prediction of response and understanding of treatment resistance. Imaging techniques of increasing sophistication hold great promise for visualization of aberrant connectivity and dysfunctional brain circuitry. The goal of practicing personalized psychiatry is now closer to becoming reality than ever before in the history of our specialty. The coauthors of this chapter have each contributed subsections commensurate with their individual expertise.