Becoming prepared to assess science learning is a daunting endeavor. Expert science teachers assess a wide range of scientific knowledge and practices, modify instruction based on assessment information, and consider the needs of their diverse learners. These skills are informed by but not necessarily consistent with what one believes and knows about assessment. In this study, I explored the ways in which the assessment expertise of 11 secondary science preservice teachers changed during their 12-month teacher education program. Conceptually, I framed assessment expertise along three dimensions: (a) constructing coherently linked assessments (Construction), (b) using assessment to support students' science learning (Use), and (c) equitably assessing English learners (Equity). Informed by the Construction-Use-Equity (CUE) framework, I designed a set of activities to support teacher learning about assessing science in linguistically diverse classrooms and delivered these activities in three of the participants' courses. Given this context, I report on both conceptual development and empirical findings from the study. First, via a scoring rubric, I articulate how the multidimensional CUE framework translates into observable levels of assessment expertise. Next, I report on the mixed methods analysis of surveys, interviews, and teacher products. Broadly, descriptive and non-parametric statistical analyses indicate that the teachers' assessment expertise changed differently among the three conceptual dimensions. Qualitative analyses describe the nature of these changes. Most notably, the teachers (a) considered the alignment between assessment task and learning objective, (b) expanded their repertoire of assessment tasks and general assessment strategies, (c) moved toward a formative view of assessment, and (d) became increasingly aware about the role of language in assessment. Finally, in greater detail, I explore the latter pattern by incorporating qualitative analysis of three teachers' classroom practices. These resulting case studies illustrate tensions about whether to assess language in addition to science content and whether to scaffold language use in assessment rather than reduce the use of language. By furthering a conceptualization of assessment expertise and by identifying and describing changing expertise, the study aims to inform how teachers are prepared to assess science.