**1. Introduction**

Depression and anxiety are frequent mental disturbances that seriously affect human life. They are commonly encountered in clinical practices, but comorbidity is often and differentiation of depressive and anxiety states is important with respect to therapeutic processes [1]. Psychiatric diagnosis at present is mostly based on psychological and behavioral assessments [2]. Additional use of objective measures as biomarkers to evaluate the symptoms and to make the differential diagnosis will be useful for the adequate treatment of depression and anxiety. A biomarker is defined as an indicator of biological and pathogenic processes or responses to exposure and intervention and is classified as molecular, histological, radiographical, or physiological index [3]. Various types of biomarkers for depression and anxiety have been studied, including genetic, biochemical, hormonal, imaging, and electrophysiological measurers, and some studies tried to use them to differentiate depression and anxiety [4]. Among physiological measures, the present chapter

focuses on heart rate variability (HRV) that has been utilized for analyzing autonomic activities to characterize and differentiate depression and anxiety.
