This paper presents case studies of four students' experiences with writing in a fourth/fifth grade classroom. The paper focuses on the following questions: What is the relationship between children's social and intellectual identities and their successes or struggles in writing? How do writing instruction and assessments mediate that relationship? The theories and research literatures that frame our study include sociocultural theories, theory and research on identity and its relationship to learning, research on writing curriculum and children's classroom experiences in writing and theories of discourse and positioning. The data analyzed for this paper were gathered as part of a two-year, qualitative, classroom-based study of children's experiences across literacy and mathematics in a fourth/fifth grade urban classroom. Data for this paper was drawn from the study's first year and included fieldnotes of observations, audio and videotapes of lessons and discussions, audiotaped interviews with all students in the classroom, and written artifacts. The first year of the study focused on twenty-three children in a large northwestern city. Participating children included Native American, African American, white, and Asian American families who have lived in the U.S. for two or more generations, as well as children who had more recently emigrated from Africa (primarily Ethiopia, Eritrea, and Somalia), Southeast Asia, Pakistan, and Mexico. Students' families were primarily working class and lower middle class with a few students living in poverty. We employed grounded theory and discourse analysis to analyze data and constructed case studies by sorting all classroom data by each focus child and then examining transcripts and other documents that provide context for each child's learning and achievement. Our findings show how various elements of the writing curriculum worked to facilitate or hinder each child's success. Our findings emphasize tensions that are illustrated in each case study. These include tensions between: common elements of a 'progressive' writing curriculum and children's individual approaches to writing; children's perceptions of themselves as writers and their performance in writing; the continually shifting discourses about writing within the classroom and the fixed notions of writing in state assessments; the need to master certain elements of "school writing" and the children's own sense of what was engaging or appropriate writing. Our case studies point to the need to increase rather than decrease flexibility in the writing curriculum and point to the complexities inherent in attempting to construct writing assessments that provide a valid measure of children's writing achievement.