The purpose of this thesis is to develop knowledge about young individuals’ reasoning and how they justify their standpoints concerning trustworthiness and decision-making in issues connected to health where available information is contradictory or uncertain. This purpose has been addressed in three different steps. In the first step almost 300 students in Swedish upper secondary school answered a web-based questionnaire, which had different types of multiple choice questions about pseudoscience and science. The results demonstrated large differences in acceptance between the different pseudoscientific statements. But there was no statement where the majority of the students agreed with the statement. There was no apparent relationship between the students’ pseudoscientific beliefs and their factual knowledge about the human body. However, the analysis revealed that students who have taken three or more science courses in upper secondary have relatively lower faith to pseudoscientific ideas. The results did not indicate any sex difference with regard to strength of faith in pseudoscientific ideas. In the second step, first year students from the science programme were observed and video-taped during two lessons, while discussing different explanation models in health. They worked in peer-groups with three to five students. The students discussed two different cases which contained a question and then two proposed answers that differed a great deal from each other with respect to scientific level. The results demonstrated that the students used four different types of epistemological resources; relativistic, normative, authoritative and scientific, when supporting their arguments about trustworthiness. No student clearly used resources from pseudoscience. The use of scientific epistemological resources was rare. Instead normative or authoritative resources appeared to be more available or more context appropriate for the students in this study. The study also demonstrated that students were able to use different epistemological resources in different situations, for example when the teacher joined the discussion and put some challenging questions to the group. In the third step, seven teenagers, 17-19 years old, participated in a video diary study and an individual interview. Four girls and three boys documented their decision-making about the new influenza and vaccination against it. The data collection was thus mainly performed outside school, in everyday life surroundings, when the teenagers justified their decision about the vaccination. The different statements and answers were categorised using discourse psychology. The categorised repertoires were of two main types; experienced emphasis and important actors. The first category comprised risk, solidarity and knowledge. In the second family and friends, media, school and society were included. The school repertoire was seldom used by the students, indicating that school and science education are not available interpretative repertoires in this context. The results demonstrate the difficulties for the teenagers to use science knowledge, in the format of correct facts or concepts. However, at the same time the results demonstrate presence and reasoning concerning the importance of scientific knowledge. This scientific discourse seems to be important when teenagers reason, make decisions and justifies their decisions in health issues. The results also raise methodological questions concerning how to investigate scientific literacy. Video diaries are suggested as an appropriate data collection tool to investigate scientific literacy in an out-of-school context. With the use of video diaries, the possibilities to investigate everyday life and decision-making go beyond the classroom.