The heart continuously and cyclically communicates with the brain. Beyond homeostatic regulation and sensing, recent neuroscience research has started to shed light on brain-heart interactions in diverse cognitive and emotional processes. In particular, neural responses to heartbeats, as measured with the so-called heartbeat-evoked potential, have been shown to be useful for investigating cortical activity processing cardiac signals. In this review, we first overview and discuss the basic properties of the HEP such as underlying physiological pathways, brain regions, and neural mechanisms. We then provide a systematic review of the mental processes associated with cortical HEP activations, notably heartbeat perception, emotional feelings, perceptual awareness, and self-consciousness, in healthy subjects and clinical populations. Finally, we discuss methodological issues regarding the experimental design and data analyses for separating genuine HEP components from physiological artifacts (e.g., cardiac field artifact, pulse artifact) or other neural activities that are not specifically associated with the heartbeat. Findings from this review suggest that when intrinsic limitations (e.g., artifacts) are carefully controlled, the HEP could provide a reliable neural measure for investigating the brain-viscera interactions in diverse mental processes.