Age is the greatest risk factor for most major causes of death and disability in developed nations. Although many aspects of aging are shared, the rate and order of various functional declines and onset of disease can vary greatly among individuals. The mechanisms underlying individual trajectories of aging are influenced by a complex combination of genes, environment and lifestyle that remains poorly understood. Most of what we know about the biology of aging comes from laboratory studies of inbred, lab-adapted species–yeast, worms, flies, and mice. While these laboratory models have facilitated rapid progress in identifying conserved mechanisms of aging, translation has been limited by the challenge of identifying causal determinants of aging in the real world. Companion animals (pets) that live in the human environment provide a unique opportunity to better understand how genes and environment shape aging outside of the lab and to test whether geroscience interventions can increase lifespan and healthspan in this context. Companion dogs, in particular, have several advantages in this context: they age biologically 7-10 times faster than people, suffer from most of the same age-related functional declines and diseases, are highly socially relevant, and allow for rapid FDA (Center for Veterinary Medicine) approval of new geroscience interventions that modify lifespan and healthspan. The primary challenge of studying aging and developing geroscience therapies in companion dogs is the lack of suitable baseline information about normative aging in dogs and lack of infrastructure to facilitate geroscience clinical trials. The Dog Aging Project is an open science, long-term longitudinal study of aging in tens of thousands of companion dogs that seeks to address these challenges. The objectives of this study are to identify the genetic, environmental, and lifestyle factors that influence aging in dogs, to discover the underlying molecular mechanisms by which they do so, and to use the insights gained to increase the duration of healthy lifespan in dogs and people.