Includes bibliographical references and index.

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The internet has radically changed the way people obtain and interact with information about diseases, treatments, and conditions. Yet, our understanding of how people access and use health information to make decisions- in other words, their health literacy- has not progressed. The overall aim of this thesis is to assess the extent to which health literacy is a valid and useful construct for policy and practice related to online health resources. A mixed-methods research programme of five studies was undertaken, influenced by realist evaluation methodology. First, to ascertain engagement with user-generated online health content (UGC) in the UK, analysis of a large European survey was undertaken. Then, the uncertainty regarding the relationship between health literacy and outcomes was addressed by a systematic review and qualitative analysis of health literacy measures. Results of these two studies informed interviews carried out with 13 'key informants': policymakers and primary care clinicians in the UK with a particular interest in health literacy and/or online information. A systematic review, incorporating a traditional narrative review and a realist review, evaluated existing trials addressing how effects of online resources vary by health literacy level. Finally, data were analysed from a feasibility randomized controlled trial, comparing usage and outcomes of accessing a 'personal experiences'-based asthma website (representing curated user-generated content) versus a 'facts and figures'-based website. Participant health literacy was assessed using an index identified from the systematic review of measures, and website usage was tracked. Approximately 25% of UK internet users engage with UGC at least monthly. The most frequent users were younger, more likely to be male, and to be carers for someone with a long-term illness. Three themes were identified from health literacy measurement: 'appropriate health decisions', 'ability to obtain healthcare services', and 'confidence'. Key informants noted the lack of clarity about how health literacy influences outcomes, and suggested that personal preferences and digital access and skills may be more relevant than health literacy for policy and practice. Existing trials of online resources in which participant health literacy was measured were mostly at high risk of bias; some possible explanations of how these interventions should work in people with low health literacy were that they may experience higher data entry burden related to chronic diseases, and that they may prefer simulated face-to-face communication. Finally, there were no differences between health literacy groups in the feasibility trial regarding usage or outcomes related to either the 'facts and figures' or 'personal experiences' websites. Taken together, these results question the validity and appropriateness of health literacy as a key objective or consideration in the development or use of online resources. While health literacy has value as a general idea, this thesis demonstrates that it may no longer be the right construct to guide intervention development and implementation to improve health outcomes.