<p>The design of inclusive mathematics instruction challenges teachers to adapt learning opportunities and activities to the individual learning requirements of the students. The knowledge about tasks as well as the adaptation of tasks that are sensitive to individual differences is of high practical relevance. However, there is a lack of empirical evidence on what knowledge teachers have in the context of adapting tasks and how teachers can be supported professionally to further develop appropriate knowledge in the context of professional development. These questions are being investigated in the project “Designing Collaborative Learning Environments” (DCLE), which is based on a blended learning in-service teacher training series for the planning and implementation of inclusive mathematics instruction. This study employs qualitative content analysis of written pre- and post-vignettes from teachers on adapting tasks for inclusive mathematics instruction, demonstrating that a blended learning format, combining face-to-face and e-learning, effectively supports teachers’ knowledge development in task adaptation. Finally, the study contributes to the theoretical operationalization of knowledge about adapting tasks in inclusive mathematics instruction.</p>