This article explores ways in which learners' achievements can be assessed and reflected in both the domains of linguistic performance and subject-content learning. The content and language integrated learning (CLIL) approach adopted by the authors is one in which language learning and content learning are truly integrated and where neither dominates the other. Due to the dual focus on language and content, CLIL assessment needs to account for the goals and objectives of two different subjects, including knowledge, competences, skills, attitudes and discourse practices, for both language and content. The CLILA project (CLIL Learner Assessment) has developed an enquiry and assessment tool to measure primary school pupils' content and foreign language ability in CLIL lessons. The tool is based on: (1) the Common European Framework of Reference and the lingualevel model; (2) the description of competences in the subject content areas; and (3) the curricula of the subjects involved with their thematic categories. With the help of CLIL teachers, descriptors, evaluation scales and examples of assessment tasks were developed and tested in several classrooms. The article outlines the theoretical background of the assessment tool and reports on the development and improvement of the assessment materials during the trial phase.