For more than 40 years the development of Computer Aided Design (CAD) tools has been focused on the description of the geometry of products. More recently, CAD tools have evolved in tools to support the Product Lifecycle Management (PLM), which are more oriented to support the management aspects of the product development process than the design process itself. Recently, it has been introduced a new design method that adopts a top-down approach, which starts from the definition of a Functional MockUp (FMU) allowing to simulate the overall behavior and the use of the concept level before the detailed design. This method is closer to the typical logical sequence of design, where the designer has at first an overall view of a system and of its sub-components, and then he takes care of the details. This method is supported by commercial tools, as the LMS-Amesim suite, or by open-source software tools based on the Modelica language. This is an open-source language allowing designers to integrate and describe at functional level several aspects of a system, including mechanical, electrical, thermal, hydraulic, control and others allowing to simulate all together. The paper analyses this methodological approach and presents some applications where some systems are designed using a functional modeling approach.