Conformal radiation therapy using dynamic beam delivery systems like scanned ion beams requires concise quality assurance procedures for the complete treatment planning process. For the heavy ion therapy facility at GSI, Darmstadt, a quality assurance program for the treatment planning system (TPS) has been developed. It covers the development and updating of software, data protection and safety, and the application of soft- and hardware. The tests also apply to the geometrical precision of imaging devices and the geometrical and dosimetrical verification of dose distributions in different phantoms. The quality assurance program addresses acceptance and constancy tests of the treatment planning program. Results of the acceptance tests served as a basis for its governmental approval. Two main results of the acceptance tests are representative for the overall performance of the system. (1) The geometrical uncertainty that could be achieved for the target point definition, setup accuracy, field contouring, and field alignment is typically 1.5 mm. The uncertainty for the setup verification using digitally reconstructed radiographs (DRR's) is limited to 2 mm. (2) The mean deviations between measured and planned dose values is 3% for standardized cases in a water phantom and up to 6% for more complicated treatment configurations.