Self-assessment is a complex metacognitive process with widely reported benefits for students in secondary education. This paper explores the impact of self-assessment on the learning and motivation of a year 12 physics class in a Cambridgeshire secondary school over a sequence of lessons covering the topic of energy. The use of self-assessment appeared to benefit students by providing them with timely, insightful feedback and promoting self-reflection and self-regulation, whilst also enabling differentiation and promoting mastery. Incorporating self-assessment in homework emerged as an opportunity for developing future practice and research, in particular using online platforms that enable students to guide their own learning.