The construction of a design model is a critical phase in real-time systems (RTS) development as the choices made have a direct impact on timing aspects. In traditional model-based approaches, the design relies largely on the designer experience. Once the design model is constructed, a convenient schedulability test has to be found in order to ensure that the design allows the respect of the timing constraints. This late analysis does not guarantee the existence of a test for the given design and does not allow early detection of unfeasible designs. In order to overcome this problem, this chapter proposes the first UML/MARTE methodology for schedulability-aware real-time software design models construction. 6.1 Introduction Model-based approaches for real-time systems (RTS) development aim at going from requirements specification to binary code production with the insurance of respecting the functional and non-functional requirements of the system. These model-based approaches (e.g. [61, 108, 229]) introduce a number of intermediate models between the requirements and the binary code. Requirements are usually formalized with use case scenarios. Even when modeled with other formalisms, critical scenarios that represent the system response to external stimuli are specified along with the system response deadlines. The functional model aims at representing functional blocks and their interactions and to show how functional blocks participate in above defined scenarios. The functional model is then refined in a design model that introduces the mech-anisms/patterns for the realization of the functional model on the underlying platform. Actually, the design model defines the architecture of the system still being independent of specific platforms. To this end abstracted resources and services are CEA, LIST,