Health Literacy (HL) refers to personal competencies for accessing, understanding, appraising, and applying health-relevant information in order to make sound decisions in everyday life that promote the prevention of bad and maintenance of good health. Schools are seen as promising intervention settings for promoting these skills in adolescents. School testing that reflects health-related tasks that students need to complete in daily life stands the best chance of promoting growth in HL. Testing can facilitate the transfer of learning, i.e. the application of previous learning in the solution of novel problems in different contexts. This study presents a newly developed performance-based instrument for the measurement of HL knowledge and skills in adolescents in a school-based context, as well as the results of a validation study. The instrument aims to measure adolescents' generic HL and takes account of competency-based principles of education. The results of the validation study prove the reliability, measured by Cronbach's alpha, and construct validity (expert panel and factor analysis) of the newly designed instrument. The study also demonstrates that the instrument can distinguish between weaker and stronger students.