Understanding of NAD+ metabolism provides many critical insights into health and diseases, yet highly sensitive and specific detection of NAD+ metabolism in live cells and in vivo remains difficult. Here, we present ratiometric, highly responsive genetically encoded fluorescent indicators, FiNad, for monitoring NAD+ dynamics in living cells and animals. FiNad sensors cover physiologically relevant NAD+ concentrations and sensitively respond to increases and decreases in NAD+. Utilizing FiNad, we performed a head-to-head comparison study of common NAD+ precursors in various organisms and mapped their biochemical roles in enhancing NAD+ levels. Moreover, we showed that increased NAD+ synthesis controls morphofunctional changes of activated macrophages, and directly imaged NAD+ declines during aging in situ. The broad utility of the FiNad sensors will expand our mechanistic understanding of numerous NAD+-associated physiological and pathological processes and facilitate screening for drug or gene candidates that affect uptake, efflux, and metabolism of this important cofactor.