Although it has been known for over three decades that mitochondria are end
owed with a complex array of Ca2+ transporters and that key enzymes of mito
chondrial metabolism are regulated by Ca2+, the possibility that physiologi
cal stimuli that raise the [Ca2+] of the cytoplasm could trigger major mito
chondrial Ca2+ uptake has long been considered unlikely, based on the low a
ffinity of the mitochondrial transporters and the limited amplitude of the
cytoplasmic [Ca2+] rises. The direct measurement of mitochondrial [Ca2+] wi
th highly selective probes has led to a complete reversion of this view, by
demonstrating that, after cell stimulation, the cytoplasmic Ca2+ signal is
always paralleled by a much larger rise in [Ca2+] in the mitochondrial mat
rix. This observation has rejuvenated the study of mitochondrial Ca2+ trans
port and novel, unexpected results have altered long-standing dogmas in the
held of calcium signalling. Here we focus on four main topics: (i) the cur
rent knowledge of the functional properties of the Ca2+ transporters and of
the thermodynamic constraints under which they operate; (ii) the occurrenc
e of mitochondrial Ca2+ uptake in living cells and the key role of local si
gnalling routes between the mitochondria and the Ca2+ sources; (iii) the ph
ysiological consequences of Ca2+ transport for both mitochondrial function
and the modulation of the cytoplasmic Ca2+ signal; and (iv) evidence that a
lterations of mitochondrial Ca2+ signalling may occur in pathophysiological
conditions.