Decreased mitochondrial membrane potential (ΔΨM) has been found in a variety of aging cell types from several mammalian species. The physiological significance and mechanisms of the decreased ΔΨM in aging are not well understood. This review considers the generation of ΔΨM and its role in ATP generation together with factors that modify ΔΨM with emphasis on mitochondrial membrane permeability, particularly the role of a multiprotein membrane megapore, the mitochondrial permeability transition pore complex (PTPC). Previous data showing decreased ΔΨM in aged cells is considered in relation to the methods available to estimate ΔΨM. In the past the majority of studies used whole cell rhodamine 123 fluorescence to estimate ΔΨM in lymphocytes from mice or rats. Imaging of ΔΨM in living, in situ mitochondria using laser confocal scanning microscopy offers advantages over whole cell measurements or those from isolated mitochondria, particularly if several different potentiometric dyes are employed. Furthermore, high resolution imaging of the newer fixable potentiometric dyes allows immunocytochemistry for specific proteins and ΔΨM to be examined in the same cells or even the same mitochondria. We found that decreased ΔΨM in p53 overexpression-induced or naturally occurring senescence is associated with decreased responsiveness of the PTPC to agents that induce either its opening or closing. The decreased PTPC responsiveness seems to reflect, at least in part, decreased levels of a key PTPC protein, the adenine nucleotide translocase. We also consider the possible basis for decreased ΔΨM in fibroblasts from patients with Parkinson’s disease, an age-related neurodegenerative disease. Finally, we speculate on the mechanisms and functional significance of decreased ΔΨM in aging.