The article focuses on the planning and implementation of interim assessment tasks in Key Stage 3 Geography. This research identifies three key dimensions which impact on the planning of assessment in the medium and long term. These are: teachers' emerging conceptualization of "formative assessment"; the statutory requirements for assessing pupils' progress and attainment in the Geography National Curriculum in Key Stage 3; and teachers' professional craft knowledge. By considering the impact of the Geography National Curriculum level descriptions on the planning of formative assessment, the article also examines the tensions between the formative and summative functions of assessment. (Contains 1 figure.)