Over the last decade, researchers in our laboratory have engineered and developed several series of genetically encoded voltage-sensitive fluorescent proteins (VSFPs) by molecular fusion of a voltage-sensing domain operand with different fluorescent reporter proteins. These genetically encoded VSFPs have been shown to provide a reliable optical report of membrane potential from targeted neurons and muscle cells in culture or in living animals. However, these various reporters also exhibit discrepancies in both their voltage-sensing and targeting properties that are essentially related to the intrinsic characteristics of the fluorescent reporter proteins. It is therefore important carefully to select the sensor that is most appropriate for the particular question being investigated experimentally. Here we examine the current state of this subfield of optogenetics, address current limitations and challenges, and discuss what is likely to be feasible in the near future.