The quality management of software models is an important issue. As a preparative task, the quality circle requires quality planning. After that a software model can repeat the sequence: quality measurement, quality analysis, and quality improvement. Until now, existing tools lack support for all these activities at once. Therefore, we developed and implemented concepts that provide the full quality circle for software models. The considered models are mainly represented in the syntax of UML class and statechart diagrams and their semantics are formally defined. The formal semantics of the considered software models allows us to improve them while preserving their external behavior.