The authors characterized changes in the prevalence of alcohol, tobacco, and marijuana use over time, and examined age of onset, co-morbid use and sociodemographic factors associated with ever using alcohol, tobacco, or marijuana in a nationally representative sample of Canadian youth. Data were collected from students in grades 7-9 as part of the Canadian Youth Smoking Survey (n = 19,018 in 2002; n = 29,243 in 2004). Descriptive analyses examined age of onset, co-morbid substance use and changes over time. Logistic regression models were used to examine factors associated with ever trying alcohol, tobacco, or marijuana with the 2004 data. Alcohol was the most prevalent substance used by youth and it was also the only substance which exhibited increased rates of use between 2002 and 2004. Co-morbid substance use was common, and it was rare to find youth who had used marijuana or tobacco without also having tried alcohol. As expected, youth who had poorer school performance were more likely to drink and smoke marijuana or tobacco, as were youth with more disposable income. Such timely and relevant data are important for guiding future policy, programming, and surveillance activities.