Why study a population's health literacy? As early as in 1979, Thomas McKeown highlighted the importance of behavior for understanding morbidity and mortality (McKeown, 1979). When studying the evolution of health and disease across the 20th century, he pointed out that most prevalent diseases (and mainly chronic diseases) were highly influenced by individual behaviors. Since then, and especially during the last decade (mainly due to keystone papers such as Health for All, Health of the Nation and Ottawa Letter), health professionals have dedicated serious efforts and time to promote health by trying to get the patients to adopt and maintain salutogenic lifestyles (and, in complement, trying to get the patients to change pathogenic behaviors such as smoking, alcohol drinking, hyper-caloric diets, etc.). A main strategy for any health promotion / disease prevention program is health education (Ogden, 2000). This is also taken into account by the several models that have been proposed to predict health behavior, in which health beliefs usually appear as a main determinant. The logic behind this is that since health beliefs are one of the predictors of health behavior, transforming wrong beliefs into accurate knowledge raises the possibility of people developing healthy attitudes and, consequently, adopting healthy behaviors and life-styles. Although scientific research has shown that health beliefs are not a sufficient condition for adopting healthy behaviors, it is consensual that health knowledge is a necessary condition, since, as stated by Sarafino, "people who want to live healthful lives need information - they need to know what to do and when, where and how to do" (Sarafino, 2002, p.188). In sum, all main predictive/explicative models of health behaviors adoption include the dimension of health beliefs (and health knowledge) together with other determinants such as self-efficacy for engaging in the 'new' behavior, perception of vulnerability to a health problem, severity of such problem, etc. Another aspect of health promotion actions is how to transmit health information in a way that maximizes the probability of adoption of healthy behaviors by individuals and populations. This is not an easy thing to do and it is especially relevant when cost-benefit analyses of a health-information campaign are to be done. At this level, several variables play important roles. A fundamental one is the source of health information. Besides formal learning (in schools, faculties, etc.), mass media and the Internet are known to be major players in health information delivery. When used for dissemination of evidence-based health-promoting information, these sources for health information can play a very useful role. Another very important source of health promotion are health professionals, namely in medical settings or through active health information campaigns conducted by these professionals. Additionally, health information transmission is known to be more effective when its contents are tailored for each specific target person or population (Kreuter, Strecher, e Glassman, 1999), and when it is message-framing oriented - i.e., emphasizing the benefits (gains) or costs (losses) associated with a behavior or decision. Evidence shows that the best type of message-framing (focusing on gains or on costs) depends on the type of behavior to change (Rothman e Salovey, 1997). Other factors that increase the effectiveness of health information are (a) the use of specific instructions for pursuing a healthy behavior (instead of only using a frightening/negative message, for example) and (b) the use of motivational statements, bolstering self-confidence of targeted people in order to implement the desired behavior (Sutton, 1982). What about Portuguese population health literacy? According to the Portuguese National Health Plan 2004-2010, health education is a strategic tool for health promotion and schools are a main setting for health education. As a matter of fact, the importance of schools in health promotion through health education has been recognized since 1994, when Portugal integrated the WHO project Health Promoting Schools. Also, the WHO document Health for all in the 21st century, endorsed by Portugal, included as a main community health target that 95% of children should get health education from schools. At this age level, health subjects that have received the most attention have been: food/nutritional habits, physical activity habits, consumption of drugs, tobacco and alcohol, sexuality and sexually transmitted diseases, mental health, and violence in schools. More recently, at the European Health Forum Gastein 2004, it has been formulated that "In a Europe of the future, Everybody has easy and prompt access to affordable, high quality health care - whoever and wherever they are... people will have no trouble finding clear and reliable information on how to be in good health and about diseases and treatment options." This statement clearly puts health literacy improvement as a priority for the European Union (and, therefore, for Portugal). In spite of this political acknowledgement (and related decisions/actions) regarding health promotion through the increase of health literacy (i.e., supporting health literacy as patients' empowerment), the fact is that there are very few studies that have attempted to give a global picture of the Portuguese population's knowledge on health (Cabral e Silva, 2009). As a matter of fact, most of the studies that were done about Portuguese health literacy are local/regional, and too health-domain-specific.