Introduction: Health-related websites have developed to be much more than information sites: they are used to exchange experiences and find support as well as information and advice. It is important that health professionals and website developers understand how content may impact users. This thesis documents the development and application of a tool to measure the impact of using health-related websites which contain experiential and/or factual information. Methods: A multi-method study with five stages. Stage 1: Questionnaire items based upon themes relating to the impact of using health-related websites were constructed following qualitative secondary analysis of 93 interviews relating to patient and carer experiences of health and a recent literature review. Items were assessed by an expert panel. Stage 2: Cognitive interviews were carried out to confirm acceptability of items. Stage 3: Item reduction steps were used to reduce the number of items. Stage 4: The validity and reliability of the remaining items were tested using traditional and modern psychometric methods. Stage 5: The new questionnaire was piloted in a randomised controlled trial. Results: Eighty-two items were constructed according to the key themes identified in Stage 1. Following expert and patient refinement, two independent item pools entered psychometric testing. The first item pool related to general views of using the internet in relation to health and the second item pool related to the impact of using a specific health-related website. Sub-scales and summary scores were found to have high construct validity, internal consistency and test-retest reliability. The questionnaire showed high completion rates and low counts of missing data in a trial setting. Conclusion: Analysis confirmed good psychometric properties in the eHIQ-Part 1 (11 items) and the eHIQ-Part 2 (26 items). Preliminary findings of trial data demonstrate the acceptability and feasibility of including the eHIQ in randomised controlled trials. This tool will enable the measurement of the impact of health-related websites containing various styles of information and support across a range conditions and facilitate their accurate evaluation in clinical trials.