Commonly used voltage sensitive dyes are substituted aminonaphthylethenylpyridinium (ANEP) dyes, such as di-4-ANEPPS, di-8-ANEPPS, and RH237. Depending on their chemical modifications which change their physical properties they are used for different experimental procedures.[1] They were first described in 1985 by the research group of Leslie Loew.[2] ANNINE-6plus is the latest voltage sensitive dye with fast response (ns response time) and high voltage sensitivity. It has been applied to measure the action potentials of a single t-tubule of cardiomyocytes by Guixue Bu et al.[3] A recent computational study confirmed that the ANEP dyes are affected only by the electrostatic environment and not by specific molecular interactions.[4]