Recent evidence suggests that impulsivity and sensation seeking are not stable risk factors for substance use among adolescents and early adults but rather that they undergo significant developmental maturation and change. Further, developmental trends of both personality facets may vary across individuals. In the current investigation, we used longitudinal data from ages 15 to 26 on 5,632 individuals drawn from the offspring generation of the National Longitudinal Survey of Youth to examine whether interindividual differences in intraindividual change in impulsivity and sensation seeking predicted the escalation of alcohol, marijuana, and cigarette use in adolescence and early adulthood. Latent growth curve models revealed significant individual differences in rates of change in both personality and substance use. Age-related changes in personality were positively associated with individual differences in substance-use change. Individuals who declined more slowly in impulsivity increased in alcohol, marijuana, and cigarette more rapidly, whereas individuals who declined more slowly in sensation seeking increased more rapidly in alcohol use only. Although risk for substance use across the population may peak during adolescence and early adulthood, this risk may be highest among those who decline more gradually in impulsivity.