There are three general categories of causes of the cognitive decline associated with aging: disuse, disease, and aging per se. People tend to use certain skills or abilities less with age and, thus, those skills decline due to the disuse. Physical illnesses tend to increase with age, which will tend to compromise cognitive functioning. Further, there are actual neurobiological changes with age that will contribute to deterioration of cognitive abilities. Variability of performance between different individuals within an age group increases with age due to each of these three major contributing factors to age decline. The best defense against age-related cognitive deterioration is practice. Practice tends to mitigate the effects of aging by not allowing disuse to occur. In addition, practice can overcompensate for age effects by building a larger reserve capacity to offset any real neurobiological effects of age. Practice can also lead to compensatory strategies in which alternative way of maintaining performance levels are found.