This international report is the fourth from the Health Behaviour in School-aged Children (HBSC) study, a WHO collaborative cross-national study, and the most comprehensive. It presents the key findings on patterns of health among young people aged 11, 13 and 15 years in 41 countries and regions across the WHO European Region and North America in 2005/2006. Its theme is health inequalities: quantifying the gender, age, geographic and socioeconomic dimensions of health differentials. Its aim is to highlight where these inequalities exist, to inform and influence policy and practice and to help improve health for all young people. The report clearly shows that, while the health and well-being of many young people give cause for celebration, sizeable minorities are experiencing real and worrying problems related to overweight and obesity, self-esteem, life satisfaction, substance misuse and bullying. The report provides reliable data that health systems in Member States can use to support and encourage sectors such as education, social inclusion and housing, to achieve their primary goals and, in so doing, benefit young people's health. Policy-makers and professionals in the participating countries should listen closely to the voices of their young people and ensure that these drive their efforts to put in place the circumstances - social, economic, health and educational - within which young people can thrive and prosper.