This study followed a small but diverse group of biology teachers through the first two years of the pilot for a new Advanced Level Biology course--Salters-Nuffield Advanced Biology. SNAB aims to modernise A-level Biology using real world contexts and examples as the starting point, promoting conceptual understanding rather than factual recall, encouraging active learning and developing a capacity for critical thinking and reflection. If the teachers were to implement this new curriculum as intended they would have to change some of their usual practices. The challenges which they encountered and the ways in which they responded were identified through a series of semi-structured interviews with each teacher, supported by lesson observations and interviews with students and technicians. A major constraint on changing practice was uncertainty about how to manage new approaches effectively. (Contains 1 table.)