This article considers the development in understanding of aspects of formative assessment of students' writing at Key Stage 3 (11-14 years) by a group of trainee English teachers. An "analysis and application" approach to writing development, which requires students to master categories and processes then apply them, is outlined and illustrated. A proposed sequence for teaching writing which relies on an "engagement and reflection" approach to writing development is used as a context for discussion of trainee English teachers' ability to identify strengths, achievements and areas for development in students' writing, trainees' ability to use assessment criteria productively, and trainees' ability to use assessment information to inform their own planning and teaching.