The membrane potential of a cell regulates several physiological processes. Membrane potential fundamentals are presented including electrophysiologic methods of membrane potential measurement. Voltage sensitive probes have been developed to record effects caused by membrane voltage changes. The electrical field dependent molecular effects used in optical membrane voltage determination are described. They comprise polar molecule effects, donor-acceptor pair effects (Förster-type resonant energy transfer, photoinduced electron transfer, photoinduced excited state intramolecular proton transfer), ground-state intermolecular proton transfer in genetically encoded microbial rhodopsins, and transmembrane helix strand displacement in voltage sensitive domains of genetically encoded voltage-gated ion channels and phosphatases. An overview of voltage sensitive probes is given, including voltage sensitive dyes, voltage sensitive semiconductor nanoparticles, fluorescent proteins, voltage-gated ion channels, voltage sensitive phosphatases, and microbial rhodopsins.