“Health Behaviour in Schoolchildren. A WHO Cross-National Survey” was started by researchers from three European countries. Presently, more than 10 countries within the European region of WHO are accepted as members of the project. The study is oriented towards behavioural and social science, and its basic philosophy is described as a lifestyle approach. In the first part of this article, differences between traditional epidemiological research and lifestyle research are outlined. The concept “lifestyle” is clarified, and the focuses of lifestyle, health promotion and health education research are described. It is argued that aspects of lifestyle not considered to be directly related to mortality or morbidity may be more important for an understanding of lifestyle of individuals or groups than aspects more directly related to health. Health-related habits, lifestyle and psychosocial aspects of health are chosen as criterion variables of the study. The personal and environmental context of lifestyle are the main factors on the predictor side, and the findings should also be discussed in a demographic and macrosocial context. National cross-sectional surveys (n at least 3000) are carried out regularly in an increasing number of countries. Data are collected anonymously at school, covering three age groups (mean age 11.5, 13.5, and 15.5 years). One part of the questionnaire consists of a core of questions and is repeated in every survey. The other part has a special focus which changes from survey to survey. Comparability of data across countries is given high priority. Results from the first survey (1983–84) are presented to illustrate the relative importance of country, indicators of inequality, social network, and beliefs in the harmful effects of smoking as predictors of smoking habits. The results confirm that a wide range of predictors are important. In line with the philosophy of the project, the focus of the second survey (1985–86) is on physical activity (as a positive health habit) and psychosocial aspects of health. Finally the report mentions some of the more practical and administrative problems related to this kind of cross-national research, and describes some of the limitations of large-scale cross-national surveys.