AbstractPsychiatric disorders are a leading cause of morbidity and mortality, yet their underlying pathophysiology remains unclear. Searches for a genetic cause of bipolar disorder, schizophrenia, and major depressive disorder have yielded inconclusive results. There is increasing interest in the possibility that defects in the mitochondrial genome may play an important role in psychiatric illness. We undertook a review of the literature investigating mitochondria and adult psychiatric disorders. MEDLINE, PsycINFO, and EMBASE were searched from their inception through September 2011, and the reference lists of identified articles were reviewed for additional studies. While multiple lines of evidence, including clinical, genetic, ultrastructural, and biochemical studies, support the involvement of mitochondria in the pathophysiology of psychiatric illness, many studies have methodological limitations and their findings have not been replicated. Clinical studies suggest that psychiatric features can be prominent, and the presenting features of mitochondrial disorders. There is limited but inconsistent evidence for the involvement of mitochondrial DNA haplogroups and mitochondria‐related nuclear gene polymorphisms, and for mitochondrial ultrastructural and biochemical abnormalities in psychiatric illness. The current literature suggests that mitochondrial dysfunction and mitochondrial genetic variations may play an important role in psychiatric disorders, but additional methodologically rigorous and adequately powered studies are needed before definitive conclusions can be drawn. © 2012 Wiley Periodicals, Inc.