Immune system, particularly low-grade systemic inflammation, is implicated in pathogenesis of depression, schizophrenia and other psychiatric disorders. Inflammation is thought to be a clinically relevant phenotype, as immune activation is associated with poor response to psychotropic medications. Currently, a number of RCTs are testing the efficacy 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? Could anti-inflammatory drugs be used to treat these disorders? If so, which patients are likely to benefit? Prof Khandaker will present data from population-based cohort studies, genetic Mendelian randomization analysis, and clinical trials addressing some of these issues. Pitfalls of current approaches and areas for future development will be discussed.