Voltage imaging using fluorescent voltage-sensitive dyes can be combined with other optical measurements, in particular with Ca2+ imaging, allowing for correlation of membrane potential changes with intracellular Ca2+ signals. Calibration of fluorescence voltage signals permits the comparison of membrane potential changes from different sites allowing spatial mapping of membrane potential changes. These two technical aspects enhance the capability of voltage imaging to address several fundamental problems of neurobiology. Here we discuss how to combine voltage imaging with the optical measurement of intracellular Ca2+ transients and different approaches to calibrate voltage-sensitive dye signals on an absolute scale.