This article looks at issues surrounding learner self-assessment and studies into teacher beliefs. It then goes on to present the findings of a study designed to explore teacher attitudes, beliefs, and behaviour with regard to learner self-assessment during the implementation of a revision of assessment procedures for teens aged 14-16 years. Quantitative and qualitative methods were used to discover what teachers understood by self-assessment and in what ways, if any, they had implemented this. The relationship between attitudes, beliefs, and practices was explored and some specific factors responsible for facilitating or obstructing implementation were identified. Data analysis produced significant findings that support those of other studies related to curricular innovation and more particularly the implementation of learner-centred approaches to assessment.