Heart rate variability (HRV) is the complex modification of the heart rate over time. Originally popularised as a tool to predict risk of mortality post-myocardial infarction, quantification of short-term HRV has more recently been adopted in the biobehavioral sciences to non-invasively estimate the outflow of the autonomic nervous system. In this talk, I provide an overview of HRV and its role in mental illness, which is generally characterised by reduced HRV. I will also cover how reduced HRV has important functional significance for social behaviours and psychological flexibility in response to stressors. Easy access to measuring HRV is both one of its strengths and weaknesses, so I will also discuss its responsible application in biobehavioral research.