ABSTRACT In the US, standardized testing in K–12 classrooms historically cultivated teacher perceptions of incoherencies with evidence-based practices and professional knowledge. In the last decade, changes in federal education law and widespread state adoption of the Next-Generation Science Standards (NGSS) have opened unprecedented opportunities for states to fully involve teachers in the creation of standardized science assessments. This mixed methods study examined how a cohort of K–12 science teachers perceived their professionalism, knowledge of the NGSS, and coherency among the NGSS and classroom science assessments after an intensive series of workshops creating NGSS-aligned items for the Illinois standardized science assessment. Despite tensions adhering to state requirements, teachers described a sense of professionalism and autonomy over the items produced, deeper knowledge of the standards, and changed views of assessment. Results can inform policymakers seeking to localize standard-aligned science test items while positively impacting science teachers’ perceptions and classroom practices.