Biological aging is the progressive loss of system integrity that occurs with advancing age, undermining the integrity and resilience of tissues and organ systems and driving vulnerability to disease, disability, and death. Animal experiments suggest that biological aging is is caused by an accumulation of cellular-level molecular changes, sometimes referred to as hallmarks, that, when slowed or reversed, result in increased healthy lifespan. Human translation of therapies to modify hallmarks of aging and extend healthy lifespan is a critical priority for an aging global population. To speed this translation, measurements that quantify the progress and pace of biological aging in humans are needed. The past several years have seen rapid progress toward this goal, with measurements based on micro-array assays of DNA methylation marks representing the current state of the art. My talk will review progress and challenges in this effort, share some of our work to develop a DNA methylation biomarker of the pace of biological aging, DunedinPACE, and report results from biological aging analysis of the CALERIE Trial, a randomized controlled trial of caloric restriction in healthy, non-obese midlife humans.