Attention, memory, and processing speed have thus far been shown to decline with age. That general intelligence also declines with age is a foregone conclusion, simply based on its strong correlations with attention, memory, and processing speed. Nevertheless, in the preceding chapter, we used psychometric theory, intelligence test data, and comparisons to nonhuman animals to establish connections between what may otherwise be perceived as unrelated abilities. The present chapter continues to develop two important points, both introduced in the prior chapter, but neither of which were sufficiently elaborated. First, we review a representative sample of decline within neurobiological systems supporting general intelligence. We show, in more detail than before, that the brain’s systems are senescing—its circulatory system, its axonal connections, and its neurochemical supports. Seeing the broad aging of these systems equally explains the global nature of intelligence and the corresponding global nature of its age-related decline. Second, this chapter reviews cross-sectional and longitudinal data, further documenting age-related intellectual decline and additionally showing that age-related cognitive decline, rather than being particular to select cognitive functions, is global decline in general intelligence.