This document summarizes evidence that mitochondrial insufficiency contributes to degenerative brain disorders. It discusses how mitochondria generate energy through oxidative phosphorylation but also produce reactive oxygen species as a byproduct. Accumulation of damage from these species over time may ultimately limit lifespan. The document reviews the free radical-mitochondrial theory of aging, which proposes that age-related mitochondrial decline impairs energy production and damages cells. Exogenous toxins can exacerbate this process and hasten disease. Nutrients that support mitochondrial function may help address brain energy deficits seen in various neurological conditions.