For half a decade model-based testing has been applied at Microsoft in the internal development process. Though a success story compared to other formal quality assurance approaches like verification, a break-through of the technology on a broader scale is not in sight. What are the obstacles? Some lessons can be learned from the past and will be discussed. An approach to MBT is described which is based on multi-paradigmatic modeling, which gives users the freedom to choose among programmatic and diagrammatic notations, as well as state-based and scenario-based (interaction-based) styles, reflecting the different concerns in the process. The diverse model styles can be combined by model composition in order to achieve an integrated and collaborative model-based testing process. The approach is realized in the successor of Microsoft Research’s MBT tool Spec Explorer, and has a formal foundation in the framework of action machines.