<p>There are extensive ongoing efforts to slow or even reverse human aging, such as with epigenetic cellular reprogramming, thymus rejuvenation or senolytics. In parallel, new and diverse metrics—known as biological clocks—have been discovered and shown to track the pace of aging in an individual and their organs, tissues and cells. These clocks have multiple potential use cases, including identifying people at high risk of disease, serving as a foundation for prevention or early detection, and determining whether lifestyle factors or an intervention can modulate the aging process. This review provides a critical appraisal of the progress that is being made with biological clocks and how they might ultimately help understand pathobiology, reduce the burden of disease and extend healthspan.</p>