| - The objective of this study was to design, implement and evaluate an intervention based on the theory of planned behaviour (TPB) to enhance young peoples' motivations to breastfeed/support a partner to breastfeed. Six semi-structured focus groups were first conducted with 48 13-14-year-olds from two schools in Northern Ireland. The salient beliefs elicited were subsequently used to design a TPB-based questionnaire that was then administered to 2021 13-14-year-old pupils (852 males; 1169 females) from 36 post-primary schools to identify the most important determinants of breastfeeding. The results were used to inform the design and implementation of an intervention package that was subsequently evaluated using a cluster randomised controlled trial involving 44 randomly selected schools across Northern Ireland. Questionnaires were administered to 18 intervention and 26 control schools at baseline and again at 1 and 6 months post-intervention to evaluate its effectiveness. Multi-level modelling was employed to analyse the data. The results revealed significant effects on women's intention to breastfeed, β = 0.208, t(1275) = 2.715, P = 0.007; attitudes, β = 0.223, t(1275) = 4.655, P < 0.001; moral attitudes, β = 0.231, t(1275) = 4.211, P < 0.001; subjective norm, β = 0.118, t(1275) = 2.521, P = 0.012; and knowledge, β = 0.109, d.f. (1275) = 7.843, P < 0.001. However, for men, the results revealed significant effects on only the construct of knowledge, β = 0.104, t(541) = 4.345, P < 0.001.The research has provided evidence to support the need for breastfeeding education in schools and has shown how a theoretical framework may be used to inform the design and evaluation of a health behaviour intervention.