It is well established that the immune system can modulate brain functioning and influence behavioural processes. Awareness of communication between the immune and nervous systems has, over the years, progressively heightened interest in the relationship between psychiatric disorders and immune function. By reviewing findings from studies investigating inflammation in the periphery and in the central nervous systems, we summarise here the evidence linking inflammation to the development of depression, schizophrenia and bipolar disorder. We discuss how a pathophysiological role for inflammation has now been recognised across different psychiatric disorders, at least in a significant subpopulation of patients. Finally, we discuss a possible role for these findings in the development of future diagnostic classifications of psychiatric disorders as well as of new treatment strategies. LEARNING OBJECTIVES: • Increase knowledge of the immune system and inflammatory markers • Learn about the most recent research findings linking inflammation to onset of psychiatric disorders • Understand the clinical relevance of increased inflammation across different psychiatric disorders, in particular in the context of the potential for future therapeutic strategies.