The processes of excitation–contraction (EC) coupling consume large amounts of energy that need to be replenished by oxidative phosphorylation in the mitochondria. Since Ca²⁺ activates key enzymes of the Krebs cycle in the mitochondrial matrix, it is important to understand the mechanisms and kinetics of mitochondrial Ca²⁺ uptake to delineate how in cardiac myocytes, energy supply is efficiently matched to demand. In recent years, the identification of various proteins involved in mitochondrial Ca²⁺ signalling and the tethering of mitochondria to the sarcoplasmic reticulum (SR) has considerably advanced the field and supported the concept of a mitochondrial Ca²⁺ microdomain, in which Ca²⁺ concentrations are high enough to overcome the low Ca²⁺ affinity of the principal mitochondrial Ca²⁺ uptake mechanism, the Ca²⁺ uniporter. Furthermore, defects in EC coupling that occur in heart failure disrupt SR-mitochondrial Ca²⁺ crosstalk and may cause energetic deficit and oxidative stress, both factors that are thought to be causally involved in the initiation and progression of the disease.