Microbial rhodopsin proteins enable optical control and optical readout of electrical activity in cells. We developed genetic and optical tools to perform both tasks simultaneously. We co-express a channelrhodopsin variant (for actuation) and an Archaerhodopsin variant (for readout) in the same cell. Using a digital micromirror device, we stimulate the cell in an arbitrary spatiotemporal pattern, and then we record the ensuing activity with high-speed fluorescence microscopy. Temporal interpolation techniques report the propagation of the action potential with 50 microsecond time resolution, and resolve spatially dependent effects of ion channel modulators. I will discuss applications to human iPS-derived cardiomyocytes and neurons, as well as ongoing efforts to image neural activity in vivo.