Free nicotinamide adenine dinucleotide (NAD + ) serves as substrate for NAD + -consuming enzymes. As such, the local concentration of free NAD + can influence enzymatic activities. Here we describe methods for using a fluorescent, genetically-encoded sensor to measure subcellular NAD + concentrations. We also include a discussion of the limitations and potential applications for the current sensor. Presented in this chapter are (1) guidelines for calibrating instrumentation and experimental setups using a bead-based method, (2) instructions for incorporating required controls and properly performing ratiometric measurements in cells, and (3) descriptions of how to evaluate relative and quantitative fluctuations using appropriate statistical methods for ratio-of-ratio measurements.