Addressing adolescent’s health literacy (HL) has been recognised as a strategy to empower individuals to take control of their health and promote health equity. Schools have been identified as an ideal intervention setting; however, many interventions are ineffective due to a lack of alignment with the context. The aim of this thesis was to design, develop and evaluate LifeLab: an engaging school-based HL intervention for socioeconomically disadvantaged adolescents. Study one was a systematic review of school-based HL interventions targeting socioeconomically disadvantaged adolescents, which aimed to identify effective intervention strategies. Although the review highlighted the dearth of interventions explicitly targeting HL, ‘hands-on’ learning activities; the use of peer support; and holistic approaches targeting the school environment, the parents or the local community were identified as effective strategies. Study two aimed to co-design and formatively evaluate LifeLab learning activities with, and for, low socioeconomic adolescents to inform the development of the LifeLab intervention. Co-design workshops with the adolescents provided valuable information on methods to engage young people in health education; practical considerations for implementing a HL intervention; and the health-related content that adolescents in this context feel is meaningful and important. Study three was a process evaluation assessing the acceptability, fidelity, dose delivered, reach and effectiveness of the LifeLab intervention. The results highlighted that overall, the intervention was positively perceived, particularly the out-of-school elements. The teachers highlighted some difficulties with the school-based content, which impacted the delivery of the lessons. The reach and fidelity of LifeLab were high, but the intervention did not impact the adolescent’s motivation to adopt healthier behaviours. This PhD highlights the importance of adopting a co-design approach and has developed an evidence-base for designing engaging HL experiences for young people, which can be used to inform future versions of the LifeLab intervention, as well as other school-based health interventions.