The primary function of the cardiovascular system is to ensure an adequate distribution of blood flow throughout the body to meet the energy demands of active tissues, muscles, and organs. Behavior requires energy and thus imposes demands on the cardiovascular system (Berne & Levy, 1981; Brener, 1987; Obrist, 1981). However, the cardiovascular system does not merely respond to peripheral feedback from active tissues, muscles, and organs; it also responds to efferent signals from the central nervous system to anticipate and prepare for action. Accordingly, motivational and emotional processes underlying behavior can exert an impact on cardiovascular function. This impact is largely mediated by the sympathetic and parasympathetic divisions of the autonomic nervous system. In this chapter, I review research from my laboratory and others indicating that the cardiac pre-ejection period (PEP) is a viable index of effort. This research shows that PEP is a reliable and valid measure of beta-adrenergic sympathetic effects on the heart, responds primarily to environmental conditions that require effortful active coping, and is sensitive to both environmental and behavioral uncertainty.(PsycInfo Database Record (c) 2023 APA, all rights reserved)