Language assessments are increasingly being leveraged for the purposes of educational reform and accountability, yet relatively little is known about how teachers respond to their implementation or what impact they have on the learning outcomes of students. Drawing on theories of educational innovation, this mixed methods study explores the interplay between testing, teaching, and learning in an urban Midwestern K-12 world languages program. Specifically, it investigates the washback effect of the American Council on the Teaching of Foreign Languages (ACTFL) Assessment of Performance toward Proficiency in Languages (AAPPL), and the potential for its influence on teachers’ practices to complement strategic efforts to introduce and manage curricular innovation at the local level. The study was divided into three distinct but complementary phases. In Phase 1, teachers (n=47) responded to an online survey addressing their background, instructional practices, and the potential influence of the AAPPL therein. In Phase 2, semi-structured interviews were conducted with a subset of teachers (n=12) in order to more richly describe their instructional and assessment-related practices. Finally, in Phase 3, AAPPL scores were obtained for all students (n=846) who took the assessment. Quantitative analyses predominantly involved the use of descriptive statistics to depict teachers’ practices and students’ performance on the AAPPL, as well as bivariate correlations to measure the relationship between the two. Qualitative analyses of interview data were aimed at identifying categories and themes leading to a rich description of teachers’ behaviors and activities in response to the AAPPL. In the final stages of analysis, both qualitative and quantitative data sources and analyses were synthesized in an attempt to develop a more nuanced understanding of the interplay between assessment, instruction, and learning. Findings suggested a moderately positive washback effect of the AAPPL, which was manifested through individual and collective efforts to refine curricula and instruction with the goal of further enhancing students’ language performance and proficiency development. Key findings included teachers’ increased use of ACTFL’s core documents to guide their planning, and an adjustment to the amount of instructional time devoted to the four modalities (listening, reading, writing, and speaking). This evidence for positive washback of the AAPPL was observed among teachers across languages and levels, and extended beyond those immediately implicated in its administration. Furthermore, the strength and direction of the influence was mediated by the close link between testing and teaching, individual differences among teachers, and the collaborative approach to managing the innovation. Evidence connecting teachers’ practices with student outcomes on the AAPPL was diffuse, with bivariate correlations indicating a potential relationship between the use of target language and explicit grammar instruction and students’ composite scores. This investigation has contributed to a greater understanding of how external language performance assessments may serve as a catalyst for refining and improving language teaching in K-12 contexts. Those seeking to promote the adoption of similar innovative practices should ensure that the assessment is aligned with the pedagogical goals of the curriculum and that their approach to introducing and managing change is responsive to the local context. While more research is needed in order to determine the potential influence of innovation on the learning outcomes of students (Green, 2013), world language programs should be encouraged to implement standards-based external assessments as a means by which to promote teachers’ adoption of proficiency-oriented instructional practices.