Self-efficacy is the belief in one's own ability to carry out a given task, and has been shown to be a powerful predictor of performance. Although researchers have considered self-efficacy within language learning, it remains a relatively underused and unexplored construct. This longitudinal mixed-method study set out to address this, by developing a questionnaire to measure students' English speaking self-efficacy, which was then given to first-year university students on eight occasions over the course of an academic year. Changes in self-efficacy were modeled using Hierarchical Linear Modeling, and potential predictors of change were assessed. The model showed that students grew in self-efficacy, although there were different rates of growth for individuals. Students were interviewed regarding growth in self-efficacy, and possible reasons for change. Students stated that efficacy increased as they became used to the class, but the importance of context as an influence on self-efficacy also emerged.