OBJECTIVES: To assess the presence and magnitude of social inequalities in mental health and health-related quality of life (HRQOL) in the population aged 8-18 years in 11 European countries. METHODS: Cross-sectional surveys were carried out in representative samples of children/adolescents (8-18 years) from the participating countries of the KIDSCREEN project. Mental health was assessed using the Strengths and Difficulties Questionnaire (SDQ), and HRQOL by means of the KIDSCREEN-10. Socioeconomic status (SES) was assessed using the Family Affluence Scale and parental level of education. The association between health outcomes and SES was analyzed with the regression-based relative index of inequalities (RII) and population attributable risk. RESULTS: A total of 16,210 parent-child pairs were included. The SDQ showed inequalities in mental health according to family level of education in all countries (RII = 1.45; 1.37-1.53). The RII for HRQOL was 2.15 (1.79-2.59) in the whole sample, with less consistent results by age and country. CONCLUSIONS: Socioeconomic inequalities in mental health were consistently found across Europe. Future research should clarify the causes of these inequalities and define initiatives which prevent them continuing into adulthood.