Self-access centers are a popular type of language support offered by an increasing number of schools and universities around the world. Assessment of the learning that takes place in such centers is seen as both crucial and problematic. Many barriers to successful assessment appear to exist, but these have not been comprehensively documented, making it difficult to develop systematic solutions. This article presents the results of a study of 46 self-access centers in five countries (Germany, Hong Kong, Spain, Switzerland and New Zealand), investigating current assessment practice. In-depth strength-weakness analyses were conducted with the managers of each of the participating centers. These revealed a complex interaction between pedagogical and practical challenges to assessment. They also showed an emphasis on the developmental role of assessment for learner autonomy and the importance of assessing both language gains and learning skills. (Contains 1 note and 1 table.)