We propose Framework-Specific Modeling Languages (FSMLs) as a special category of Domain-Specific Modeling Languages that are defined on top of an object-oriented application framework. They are used to express models showing how framework-provided abstractions are used in framework-based application code. Such models may be connected with the application code through a forward and a reverse mapping enabling round-trip engineering. We also propose a lightweight and iterative approach to round-trip engineering. Furthermore, we present a proof-of-concept FSML for modeling the interaction of workbench parts within Eclipse. Finally, we identify a number of challenges, opportunities, and directions for future research on FSMLs.