We describe how Computer Algebra Systems (CASs) can be used as cognitive tools in a learning environment. In particular, we
show how a CAS is employed in ActiveMath, how different types of CAS-exercises are designed, and how feedback can be produced with the help of the very same CAS.
We report the results of a first preliminary formative evaluation of ActiveMath’ CAS-exercises in a university course and some modifications of ActiveMath caused by this evaluation.