Object Orientation has been used with the Simulation Model Portability 2 (SMP2) standard with the purpose to provide a design solution for the Simulators (especially Aircraft). In object orientation, the distinctive features of an object are expressed in interfaces which are used to specify the interactions between objects. In these conditions, the planning of the coding activity can be done by describing the objects and the business logic they share. If these descriptions are written before any programming language specialization, it can be called design artifacts, or design models. A different approach to the design of software is the specification of code functionalities through test code. Using only test code it is possible to design a piece of software before entering the stage of source code development. Similarly to design models, test code is a design artifact, but it does not follow a pre-defined semantic scheme as per design models. Nonetheless, test-driven development is a restrained process, carried out in closed loop through source code re-factoring. In this book two different convergent development lines are used to provide the Hybrid design solution.