Membrane voltage (Vm) is a fundamental biological parameter that is essential for neuronal communication, cardiac activity, transmembrane transport, regulation of signaling, and bacterial motility. Optical measurements of Vm promise new insights into how voltage propagates within and between cells, but effective optical contrast agents have been lacking. Microbial rhodopsin-based fluorescent voltage indicators are exquisitely sensitive and fast, but very dim, necessitating careful attention to experimental procedures. This chapter describes how to make optical voltage measurements with microbial rhodopsins.