Mixed states are complex manifestations of bipolar disorders. Pathophysiology of mixed states remains unclear. Several models have been proposed to understand the mechanisms underlying these mood states. These models describe mixed state either as a combinaison of depression and mania, as well as a transition between mania and depression, or mixed state as a severe type of depression or mania. Pathophysiological hypotheses involve temperaments or some personality disorders, psychiatric comorbidities as well as substance use disorders, or thyroid dysfunction. However, the formal demonstration of any specific genetic vulnerability to mixed state has not yet been provided.