Mitochondrial energy production is dependent upon the maintenance of the chemiosmotic gradient across its inner membrane. This gradient is known as the proton motive force (PMF). This gradient is generated by the activity of respiratory enzyme complexes in the mitochondria. The energy stored by the PMF facilitates phosphorylation of ADP to ATP by ATP synthase (complex V). The membrane potential (ΔΨm.) that arises due to the net movement of protons across the inner membrane contributes most of the energy stored in the gradient and is a good indicator of the energy state of the mitochondrion. The collapse of ΔΨm due to the opening of high conductance permeability transition pore has been linked to apoptotic cell death. Hence, there is a lot of interest in developing reliable tools to evaluate ΔΨm. Several cationic lipophilic compounds have the ability to become more concentrated in the relatively more electronegative mitochondria compared to the cytosol and can serve as useful tools to monitor changes in ΔΨm.