Purpose: To identify predictors of smoking onset and cessation between early (age 13 years) and late adolescence (age 18 years) and between late adolescence (age 18 years) and young adulthood (age 23 years). Methods: The authors employed logistic regression to predict smoking initiation and cessation for an ethnically diverse sample of 3056 adolescents recruited from 30 West Coast schools in 1985 and observed from age 13 to age 23 years. Fifty-six percent of the sample was female. Predictors tapping sociodemographic characteristics, environmental influences, attitudes and beliefs about smoking, bonds with school, and problem behavior were measured at age 13 years (older teenager models) and at age 18 years (young adult models). Results: Robust predictors of both initiation and cessation across the two developmental periods included doing poorly in middle/high school and prior smoking behavior. Predictors common to three of the four models included being young for one's grade cohort and intending to smoke in the next 6 months. Early deviant behavior and drinking fostered initiation among older teenagers, but problem behavior as an older teenager did not predict young adult initiation. Smokers who had few or no high school friends who smoked and felt able to resist prosmoking pressures at age 18 years were more likely to quit by age 23 years. Being female predicted initiation by age 18 years; being African-American, Hispanic, or Asian inhibited it. Conclusions: The strong association of prior smoking behavior and intentions with later smoking status among both adolescents and young adults underscores the importance of starting smoking prevention early and continuing it through high school. Such programs might also consider the greater vulnerability of females, youth who are young for their grade cohort, and those who are doing poorly in school.