Substance use is generally considered as one of the most important types of risk behaviour among adolescents. In order to understand substance use patterns in the adolescent population and to be able to take timely, adequate and targeted measures to prevent substance use related harm, it is essential to closely monitor factors like the prevalence of use, age of first use, intensity of use, trends, risk factors and co-morbid problems. The aim of this thesis is to provide this information for the use of alcohol and cannabis among Dutch adolescents. To this end, two high quality, nationally representative epidemiological studies, i.e. the Dutch National School Survey on Substance Use (DNSSU) (Monshouwer et al., 2004) and the Health Behaviour in School-aged Children study (HBSC) (Dorsselaer et al., 2007) were analysed. The first part of this thesis describes the prevalence and trends in the use of alcohol and cannabis. One of the main findings was that the increase in the life time prevalence rate of alcohol use over the period 1992-2003 was particularly strong among the youngest age group (12-14 years). The study on cannabis demonstrated that the changes in the prevalence rates across 1988- 2003 reflected underlying shift in the age of first cannabis use. For example, while prevalence rates showed an increase from 1988 to 1996, age of first cannabis use decreased. The second part of the thesis focussed on the progress to risky substance use patterns i.e. drunkenness and polydrug use. It was shown that that the time interval between the first drink and first drunkenness is small (1,6 years). Furthermore, polydrug use appeared to be highly prevalent, almost one quarter of the 12-16 year olds used two or more substances (including alcohol, tobacco, cannabis and hard drugs) in the month before the survey. The risk of becoming a polydrug user increased with age, was largest for ethnically Dutch students and those attending lower vocational education. The third part of this thesis showed that both alcohol and cannabis ...