Background assessment of emotional reactivity, defined as rapid emotional responses to salient environmental events, has been neglected in mood disorders. This article reviews data showing the relevance of using emotional reactivity to better characterize bipolar mood episodes. Method We reviewed clinical data on emotional reactivity during all phases of bipolar disorders (euthymic, manic, mixed and depressive states) and brainimaging, neurochemical, genetic studies related to emotional reactivity disturbances. Result Euthymic bipolar patients show mild abnormalities (hypersensitivity to emotional stimuli and higher arousability) in comparison to controls. Both manic and mixed states are characterized by a significant increase in emotional reactivity. Furthermore, emotional reactivity may discriminate between two types of bipolar depression, the first being characterized by emotional hypo-reactivity and global behavioral inhibition, the second by emotional hyper-reactivity. Brain-imaging studies can help to identify the underlying mechanisms involved in disturbances of emotional reactivity. Conclusion Emotional reactivity can be used to refine more homogeneous pathophysiological subtypes of mood episodes. Future research should explore possible correlations between biomarkers, response to treatments and these clinical phenotypes.