The European Language Portfolio (ELP) is an alternative assessment used in foreign language classes throughout Europe to support and record language learning. Directly linked to the Common European Framework of Reference for Languages (Council of Europe, 2001) proficiency guidelines, it is designed to achieve an ambitious dual goal: document students' skills and foster self-regulated learners. The question remains whether the ELP accomplishes its desired effect. To investigate it, the researcher employed an embedded mixed methods design in Saxony, Germany, comprising a total of 575 students in 28 classes with 19 teachers in 6 schools. In a preliminary two-group quasi-experimental phase, students using the ELP (n?=?318) reported higher mastery goal orientation, task value, academic self-efficacy, self-regulatory efficacy, and instructor evaluations. The ELP's effect increased with its frequency of use. Investigation into teachers' pedagogical beliefs revealed no statistical difference, strengthening the attribution of the results to ELP use. To reveal a more complete understanding of the ELP's effect, semi-structured interviews investigating student and teacher perception of the ELP were conducted with a purposefully selected subgroup of participants using the ELP. Interviews included open-ended prompts designed to illustrate the quantitative results. This study provides strong empirical evidence supporting the claim that the ELP accomplishes its pedagogical purpose.