Oxidative phosphorylation is regulated by mitochondrial calcium (Ca) in health and disease. In physiological states, Ca enters via the mitochondrial Ca uniporter and rapidly enhances NADH and ATP production. However, maintaining Ca homeostasis is critical: insufficient Ca impairs stress adaptation, and Ca overload can trigger cell death. In this review, we delve into recent insights further defining the relationship between mitochondrial Ca dynamics and oxidative phosphorylation. Our focus is on how such regulation affects cardiac function in health and disease, including heart failure, ischemia-reperfusion, arrhythmias, catecholaminergic polymorphic ventricular tachycardia, mitochondrial cardiomyopathies, Barth syndrome, and Friedreich's ataxia. Several themes emerge from recent data. First, mitochondrial Ca regulation is critical for fuel substrate selection, metabolite import, and matching of ATP supply to demand. Second, mitochondrial Ca regulates both the production and response to reactive oxygen species (ROS), and the balance between its pro- and antioxidant effects is key to how it contributes to physiological and pathological states. Third, Ca exerts localized effects on the electron transport chain (ETC), not through traditional allosteric mechanisms but rather indirectly. These effects hinge on specific transporters, such as the uniporter or the Na/Ca exchanger, and may not be noticeable acutely, contributing differently to phenotypes depending on whether Ca transporters are acutely or chronically modified. Perturbations in these novel relationships during disease states may either serve as compensatory mechanisms or exacerbate impairments in oxidative phosphorylation. Consequently, targeting mitochondrial Ca holds promise as a therapeutic strategy for a variety of cardiac diseases characterized by contractile failure or arrhythmias.