Introduction

Young people’s well-being is of special concern in all societies and there are constant efforts to reduce all types of dangerous behaviour. These include consumption of tobacco, alcohol, illicit drugs and the use of non-prescribed pharmaceuticals, as well as extensive internet use, gaming and gambling. All countries have laws in place that restrict the availability of psychoactive substances and access to gambling activities. The legal framework may vary between countries but often includes restrictions specially meant to protect young people. Moreover, major international bodies such as the United Nations and the European Union are constantly looking for policy measures to reduce the negative impact of the use of different substances, for example the global strategy to reduce the harmful use of alcohol (World Health Organisation, 2014), the WHO Framework Convention on Tobacco Control (World Health Organisation, 2005), the European drugs strategy (Council of the European Union, 2012; Culley et al., 2012), or the European alcohol strategy (COWI Consortium, 2012).

Over the years, many studies have been conducted to improve the understanding of consumption patterns. However, despite the significant number of studies conducted in many countries, it has remained difficult to obtain a comprehensive picture of substance use in different European countries, especially of patterns of use among young people. With the launch of the European School Survey Project on Alcohol and Other Drugs (ESPAD) in 1995, data on substance use and risky behaviours became available that serve as a basis for the monitoring of substance use in Europe, as well as for analysing the risk factors, protective factors and time trends of such behaviours.

The main purpose of the ESPAD project is to collect comparable data on substance use among 15- to 16-yearold students in as many European countries as possible. The target group consists of students who turn 16 during the year of data collection, which in 2015 meant students born in 1999. The surveys are conducted in schools in the participating country, during the same period of time and using a common methodology. The ESPAD project provides comparable data in databases that have been and will be used by the research community for in-depth analyses to increase the understanding of substance use among European students (see www.espad.org/en/References--Literature).

Because of its common methodology, analyses based on ESPAD data have substantially contributed to the field of substance use. For instance, studies have been conducted on validity issues (Gmel et al., 2010; Molinaro et al., 2012; Steppan et al., 2013), methodological (Thrul et al., 2016) and theoretical issues (Brunborg et al., 2014), substance use policies (Bjarnason et al., 2010; Müller et al., 2010), risk and resilience factors (Kraus et al., 2010; Vorobjov et al., 2014), polysubstance use (Kokkevi et al., 2012; Mammone et al., 2014; Olszewski et al., 2010), attitudes (Beck et al., 2014) and risk perceptions (Piontek et al., 2013). In addition, ESPAD results have been used for the development of international action plans and strategies related to alcohol and other drugs and as such have impacted on public discussion and served as a basis for policy measures and preventive activities targeting young people.

Although the comparison of cross-sectional data on substance use across similar populations in countries of various social, economic and cultural origins is important, the possibility of investigating temporal changes across the majority of European countries is quite unique. The ESPAD project provides data that can be used to monitor trends in substance use among students in Europe and to compare trends between countries and between groups of countries (Fotiou et al., 2014; Molinaro et al., 2011). With the 2015 survey, ESPAD data cover substance use behaviours of 15- to 16-year-old students over a period of 20 years. Since 1995, when information on substance use was collected in 26 countries (Hibell et al., 1997), the survey has been repeated every fourth year in the same age group. In the second wave, in 1999, data were collected in 30 countries (Hibell et al., 2000), and the surveys of 2003 and 2007 covered 35 countries each (Hibell et al., 2004, 2009), with an additional five countries collecting data in 2008. The number of participating countries in the 2011 survey was 36 (Hibell et al., 2012), with three more countries collecting data in the autumn (Hibell and Guttormsson, 2013), and 35 countries collected data in the most recent (2015) survey.