The acute uptake of calcium (Ca²⁺) into the mitochondrial matrix via the mitochondrial calcium uniporter is a key signal that enhances mitochondrial ATP production in parallel with stimuli that increase cytosolic Ca²⁺ signaling and cellular energy demand. This physiologic mitochondrial Ca²⁺ (_mCa²⁺) accumulation is critical for the heart to rapidly increase its workrate in response to sympathetic stimulation during exercise or fight-or flight responses. In excess, though, acute _mCa²⁺ loading triggers mitochondrial permeability transition (mPT) and necrotic cell death. Such _mCa²⁺-dependent mPT drives initial tissue injury and subsequent organ-level dysfunction in response to acute ischemic insults including myocardial infarction and stroke. However, the contribution of _mCa²⁺ overload and mPT to more gradually-developing dysfunction in cardiovascular diseases featuring a sustained increase in cytosolic Ca²⁺ concentration remains poorly defined. This question is particularly relevant for forms of non-ischemic heart failure that develop over time following chronic elevations in cardiac afterload and neurohormonal stimulation. These pathologies feature progressive cardiomyocyte death that can compromise the contractile function of the heart. Understanding the relationship between _mCa²⁺ accumulation, mPT, and cardiomyocyte dropout in the context of such chronic heart disease is complicated by the proposed homeostatic role of the mitochondrial permeability transition pore (mPTP) as a route for physiologic _mCa²⁺ efflux that limits deleterious _mCa²⁺ overload. This talk will highlight recent in vivo findings from genetic mouse models featuring manipulation of the mitochondrial calcium uniporter or the _mCa²⁺ efflux machinery to modulate _mCa²⁺ accumulation in experimental models of non-ischemic heart failure. It will also address the complex interplay between altered _mCa²⁺ homeostasis, the proposed constituents of the mPTP, and cell death in the chronically-stressed heart.