Trust, authority and credibility in the digital environment have attracted the interest of many researchers in the recent years. Reliable and credible information are key factors for making numerous decisions in our everyday lives, from making political choices to choosing a healthy lifestyle. Various health-related narratives are present in the public sphere. Health advice and recommendations on vaccination, antibiotics, nutrition, exercise and numerous other topics are provided by different sources, scientific community, healthcare professionals, media, lay people, patients, social networks, etc. It was the infodemic associated with the coronavirus pandemic that again brought to the forefront the importance of reliable and credible health information and health information literacy in this complex digital environment. In such fragmented information ecosystem, characterized by the post-truth era, where misinformation and fake news spread by uninformed individuals or advocates of various conspiracy theories are considered equivalent to reliable scientific information, individuals have to choose whom to trust, based on their critical capacities and competencies, in order to make health decisions for themselves and their families. The young people of today, especially those born after the year 2000, represent the generation of digital natives, who grew up and spent their entire lives surrounded by digital technologies that have become an integral part of their lives. They think and process information differently than their predecessors. Most of them use the Internet as their primary source for accessing information, most often through smartphones, spend the vast majority of time online, increasingly use social networks for private and professional purposes, but at the same time have difficulty assessing the reliability of online information. The way they assess trust in digital information is different from the way they question information credibility in traditional media. Given that trust and credibility of information strongly influence the effect of information message itself, which, in extraordinary circumstances and public health crisis such as the coronavirus pandemic, is of vital importance for the lives of people, it is very important to understand how users decide whom to trust. Health information literacy is a multidimensional and dynamic concept that incorporates social, cognitive, economic, and personal skills to search, understand, and use health information in order to improve and maintain good health. The evaluation of online health information, i.e. its reliability and credibility, plays a significant role in the process of health behavior change. Research to date has not reached a consensus on unique framework for understanding the process of building user confidence in online health information. The focus of this doctoral dissertation are theoretical and analytical challenges of health information literacy as a multidimensional and dynamic concept, particularly in understanding the process of creating trust in online health information. By examining the attitudes and opinions of medical students about the evaluation of online health information we aimed to determine factors that influence the assessment of health information quality and credibility. The main research objective is to examine the element of information appraisal in the concept of health information literacy and to determine the processes of creating trust in health information in an online environment. The specific research objectives are as follows: 1. to identify factors and techniques for assessing the quality, reliability and credibility of online health information; 2. to determine the elements of cognitive heuristics for assessing the reliability of online health information; 3. to determine the impact of infodemic during a healthcare crisis on online health information trust issues; 4. to examine methods for assessing the quality and reliability of user-generated online health information; 5. to investigate medical students' attitudes and knowledge of the concepts and tools for assessing health information literacy. Our research aimed to determine common elements of information appraisal in the concept of health information literacy and related concepts dealing with online health information (health literacy, information literacy, media literacy, digital literacy, etc.). The research was performed using multiple methods conducted in three phases: 1. literature review and comparative analysis of selected tools and instruments for assessment of health information literacy and related concepts; 2. a survey questionnaire that examined the attitudes and opinions of medical students of the University of Zagreb School of Medicine regarding online health information appraisal, and 3. semi-structured interviews with a smaller subgroup of respondents in order to build on the findings from the survey questionnaire related to the determination of factors that influence trust in online health information, i.e. assessment of their quality and credibility. The results of our research demonstrated that medical students have the knowledge, skills and techniques necessary to assess the quality, reliability and credibility of online health information. Although medical students generally considered Internet to be a questionable source for health information, the majority reported that they always use online search engines as the first step when searching for health topics, regardless of their purpose (general interest, academic assignment, or personal or health problem among family and friends, etc.). Medical students consider the following analytical criteria when assessing the credibility of online health information: trustworthiness, completeness, objectivity and comprehensiveness of information, referencing other data sources and author disclosure. Apart from analytical criteria, medical students also use cognitive heuristics when assessing the reliability of online health information, especially user-generated content that is published and shared via social networks. Students also emphasized the importance of critical thinking, factual analysis, information verification on multiple sources, determination of source credentials as well as sharing information with family members, friends, colleagues, physicians and university professors who they consider as cognitive authorities in health communication. Research results indicate an overall positive relationship between perceived health information literacy of medical students, their ability to evaluate online health information and trust in the Internet as a source of health information. Students with high self-perceived level of e-health literacy more often use websites of government institutions and professional medical associations than other sources for online health information. Our research also revealed that the degree of trust that medical students have in online sources of information affects their behavior when searching and evaluating online health information. In healthcare crisis situations, medical students apply additional evaluation criteria for usergenerated health information on social media. In general, medical students expressed mistrust towards social networks, especially as a source of credible health information in the context of an infodemic. Although the majority of students use social networks only passively, they also believe that social networks present a platform that has the potential, if used appropriately, to convey reliable information and improve health information literacy. Students also stressed that, during infodemic, strategic crisis communication skills should be applied for proactive dissemination of credible, factual and scientific evidence in order to contribute to a faster and more efficient containment of the spread of misinformation and fake news on social media. The results of our research have shown that online health information should be comprehensive, accurate, evidence-based, objective and up-to-date in order to convey a quality health message. According to medical students, quality of information, source credibility, comprehensibility and readability, as well as verification of information in evidence-based sources are main predictors of trust in online health information. Understanding which health channels and sources are considered most reliable among different user groups can help health professionals reach at-risk patients in order to communicate health messages more successfully and implement interventions aimed at strengthening skills that are important for health information appraisal. By analyzing medical students` attitudes and opinions about credibility of online health information, we explored ways to promote critical thinking in the digital environment in order to empower individuals to make decisions in their everyday life based on verified and credible sources of information. Placing greater emphasis on lifelong learning about health, and formal and informal health information literacy education, enables greater autonomy and empowerment of individuals, and can be viewed as part of personal development towards an improved quality of life. Health information literacy increases individual and community resistance to harmful and misleading information and reduces the impact of the infodemic on human health. Improving health information literacy among population requires joint action and cooperation between stakeholders and policymakers (government authorities, media, scientific, educational and health institutions, healthcare professionals, scientists, researchers, librarians and others). Health information literacy is an important competence of every individual, but its implications for society are much broader. They include personal and social responsibility of individuals and taking ethical actions for the benefit of the community. A modern democratic society implies engaged citizens who autonomously reflect on political, ethical and social challenges in everyday life. Empowerment in health and social care, achieved by developing critical competencies, helps in distancing oneself from various social pressures, making independent informed decisions and taking responsibility for one's own health. Croatia, unlike most other European countries, still does not have an official strategy or action plan for developing and improving health information literacy among its population. Our research results point out the importance of including health information literacy in the national health policy and education strategy, with particular emphasis on developing critical thinking skills and competencies in evidence-based approach to online health information appraisal. The results of this research contribute to the conceptual linking of theoretical constructs on information appraisal and critical theories from information and communication sciences with constructs from the field of public health. The mentioned approach resulted in unifying the narratives of different domains, and synthesizing separate theoretical frameworks, namely health information and related literacies, which adds to the value of this research. The results of this research serve as a basis for defining skills for critical appraisal of online health information in order to increase the level of health information literacy and development of tools for evaluating the reliability of online health information. On the applicative level, this doctoral dissertation provides recommendations for communication of reliable online health information from the current infodemic perspective, in planning public health activities and interventions in the future.