Emotions and stress are integral parts of our lives, and they enable adequate responses to changes in the external or internal milieu. Their processing and regulation involve both the central (CNS) and the autonomic nervous system (ANS). Heart-brain or ANS-CNS interactions can be quantified through heart rate variability (HRV). While the physiology of HRV is well-understood (i.e., parasympathetic cardioregulation), its psychological significance and brain-level correlates remain unclear. When acquired under (task-free) resting conditions, HRV can serve as a "trait" marker, which has been connected to several health measures. When acquired under stimulation or tasks, HRV can quantify inter- and intra-individual "state" differences in physiological responding. In addition, HRV can be measured both in the laboratory and, using ambulatory assessment, in daily life. I will present a set of studies, in which we link HRV to emotions, acute stress, and the brain. Behaviorally, we found changes in HRV during emotional and acute stress processing, which varied as a function of gender, health status, body weight, and cultural parameters. Using fMRI, we could relate inter-individual differences in resting HRV to differences (1) in brain activation during an emotional face matching task and (2) in brain connectivity at rest. Combing neuroimaging with HRV and measuring HRV both at rest and during tasks thus contributes to our understanding of the link between heart-brain interactions and emotional processing.