Health literacy measurement studies are important for the success of health promotion efforts targeting adolescents. However, the majority of health literacy measurement tools were originally developed for adult populations and may not be reflective of health literacy in the context of adolescence. The present study sought to co-design a health literacy questionnaire and vignettes for adolescents in Ireland aged 12-18 years. This article describes the qualitative phase of the study. In 2019-2021, eight adolescents from the Irish Heart Foundation's Youth Advisory Panel participated in two concept mapping workshops during which they defined healthy living. Results of the thematic analysis indicated that adolescents defined 'healthy living' as a life that was balanced, prioritizing mental health and sleep. According to their definition, healthy living was comprised of six main constructs, namely, knowledge; sources of quality health information; facilitators and barriers; influence of others; self-efficacy, self-management and resilience; and citizenship and communication. These constructs were used to develop vignettes and items for the Adolescent Health Literacy Questionnaire (AHLQ). These were tested on a sample of 80 adolescents to check whether the respondents understood the items and vignettes as intended. Results of the nine cognitive interviews indicated that the adolescents understood the content of the 10 vignettes and 41 items. While the vignettes and AHLQ were developed with Irish adolescents, the approaches taken can be generalized to adolescents living in other countries. This will allow for the development of tailored and relevant solutions for health literacy development and health promotion for this sub-population.