Immune dysfunction, particularly low-grade systemic inflammation, is implicated in pathogenesis of depression, schizophrenia, and other psychiatric disorders. Inflammation is a clinically relevant phenotype, as it is associated with poor response to psychotropic medications. Currently, several RCTs are testing the effectiveness of anti-inflammatory drugs for patients with depression and schizophrenia. However, there are key unanswered questions, both mechanistic and clinical. Is inflammation a causal risk factor for depression and schizophrenia? What causes inflammation?