Membrane potential (V) is a fundamental biophysical signal present in all cells. V signals range in time from milliseconds to days, and they span lengths from microns to centimeters. V affects many cellular processes, ranging from neurotransmitter release to cell cycle control to tissue patterning. However, existing tools are not suitable for V quantification in many of these areas. In this review, we outline the diverse biology of V, drafting a wish list of features for a V sensing platform. We then use these guidelines to discuss electrode-based and optical platforms for interrogating V. On the one hand, electrode-based strategies exhibit excellent quantification but are most effective in short-term, cellular recordings. On the other hand, optical strategies provide easier access to diverse samples but generally only detect relative changes in V. By combining the respective strengths of these technologies, recent advances in optical quantification of absolute V enable new inquiries into V biology.