Substance use is common among homeless and precariously housed youth, yet few longitudinal studies track their usage over time. This paper analyzes data from a study of 187 youth and reports on their substance usage in the preceding month, year, and over their lifetime. The results are compared within the sample by sex and against a sample of similarly located housed youth. Findings suggest that female homeless and precariously housed youth report lower substance use, but that with interventions substance use decreases for both sexes. Compared to housed youth, those who are homeless and precariously housed begin substance use at a significantly younger age.