We present a longitudinal study of novice teachers' appropriation, negotiation, and recontextualization of assessment tools and practices. During the four years of the study, we observed and interviewed beginning mathematics and social studies teachers, along with their colleagues, mentors, and supervisors, from their time in a graduate secondary teacher education program through their second year of professional teaching. Analysis of fieldnotes, interview transcripts, and artifacts suggests that assessment tools function as boundary objects in negotiations within and between the social worlds in which novices learn to teach. Boundary objects serve as reifications or representations of values, goals, and meanings (Bowker & Star, 1999; Star & Griesemer, 1989; Wenger, 1998). Assessment tools and artifacts, as boundary objects, facilitate engagement of teachers, administrators, students, and their families in coordinating activity across social boundaries and are central to the function of educational organizations. Novice teachers' motivation to learn promoted assessment tools and their affiliated practices changed as they crossed boundaries between university and school and changed positions from student teacher to newcomer to experienced novice. Implications of this analysis for understanding novice teacher learning and motivation, and for assessment policy more broadly, are discussed. (Contains 2 tables, 1 figure and 7 footnotes.)