The coenzyme NAD+ is critical in cellular bioenergetics and adaptive stress responses. Its depletion has emerged as a fundamental feature of aging that may predispose to a wide range of chronic diseases. Maintenance of NAD+ levels is important for cells with high energy demands and for proficient neuronal function. NAD+ depletion is detected in major neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases, cardiovascular disease and muscle atrophy. Emerging evidence suggests that NAD+ decrements occur in various tissues during aging, and that physiological and pharmacological interventions bolstering cellular NAD+ levels might retard aspects of aging and forestall some age-related diseases. Here, we discuss aspects of NAD+ biosynthesis, together with putative mechanisms of NAD+ action against aging, including recent preclinical and clinical trials.