This lecture covers the state of the art in optical methods for recording neural activity, including micro-electrodes, patch-clamp, and micro electrode arrays. It explores the limitations of these methods and introduces voltage-sensitive dyes for high signal-to-noise ratio voltage mapping. The lecture delves into two-photon fluorescence microscopy, explaining the principles of single and two-photon excitation. It also discusses the development of powerful techniques like random access multi-photon microscopy and simultaneous Ca²+ release and membrane potential recording. The spatiotemporal relationship between Ca²+ release and action potential in cardiomyocytes is examined, along with the use of Arch-derived voltage indicators with improved speed, sensitivity, and brightness.