Biomarkers research in bipolar disorder is a new field with an expanding knowledge. Bipolar disorder is associated with an imbalance of diverse biological systems, including immune system, neurotrophins, neuroendocrine axis and oxidative stress. Peripheral molecules related to the pathophysiological mechanisms underlying bipolar disorder may be putative biomarkers of the disease. Bipolar disorder is associated with structural and functional neuroimaging changes associated with brain atrophy, notably involving the enlargement of lateral ventricles and decrease in prefrontal cortex and amygdala volumes. Bipolar disorder peripheral and neuroimaging findings may reflect neuroprogressive processes of the illness. A single candidate biomarker seems unlikely to tap clinical and biological heterogeneity of bipolar disorder.