Dogs develop age-dependent cognitive deficits that parallel declines observed in normal and pathological aging in humans. Like humans, some dogs show no decline, some show mild decline, and others of the same age develop severe cognitive impairments. Aged dogs show decreases in locomotion and changes in other behaviors but continue to display appropriate responses, just to a lesser extent compared to young dogs. Impaired dogs by contrast, show abnormal responses. These dogs are hyperactive, unresponsive to social stimuli or toys, and often engage in stereotypical behaviors. The aged dog is also well suited for investigation of the initial stages of plaque formation in the aging brain. Beta-amyloid deposition in the form of diffuse plaques is a prominent feature of the aged dog brain. Oxidative damage and neuron loss are also features of the canine brain while neurofibrillary tangles are absent. MRI procedures also reveal commonalities between the aging dog and human brains, including shrinkage, spontaneous lesion formation, and reduced blood flow. These findings suggest that the dog can be used to model the earliest phases of cognitive, behavioral and neuropathological changes associated with brain aging in humans. The existence of dogs that maintain cognitive function with age may provide insight into promoting successful aging in humans.