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The success of rapid prototyping techniques has showed the need for means to create tangible prototypes from CAD parts. As the first polymer parts were created using pointwise solidification techniques, users inquired about the possibility of creating metallic parts. The techniques behind the rapid prototyping of metallic parts are pointwise techniques, typically featuring the melting of raw material using a laser or plasma. The possibility of changing the material so that the deposition can consist of an arbitrary mix of materials is readily available. The next generation of Rapid Prototyping Machines will allow parts made from multiple materials to be manufactured in a single process. Introducing multiple materials within a part requires the specification of the material composition within the volume enclosed by this solid. There are only a few means available for the creation of heterogeneous solids, most of them featuring a guided exploration of the design space through the use of structural and shape optimizers. Conventional solid modeling schemes (CSG, B-Rep…) fail to capture information about the material distribution inside an envelope, as the task of representing solids is oversimplified to the representation of their enclosing boundary. This research effort addresses a representation scheme for arbitrary 3D material distributions, termed MMa-Rep (for MultiMaterial Representation.) The techniques employed here are built upon conventional surface interpolation techniques extended to volumes and compositions, using a mapping from a 3-Dimensional parameter space to a hyperspace (geometry + compositions.) This dual-interpolation method presents two independent interpolation techniques that can be customized separately to meet specific needs. The constraints set on the material interpolation resulting from the material completeness requirement are exposed, and their impact on the selection of a valid material interpolation method is discussed. This representation scheme is then evaluated in the context of emerging design methodologies used for heterogeneous solids. The natural extensions to conventional representation techniques such a Constructive Solid Geometry (CSG) using MMa-Rep and to the representation of heterogeneous solids typically resulting from structural or material optimization processes are studied. As the design of solids featuring varying material distributions inside their boundaries is new, volumetric visualization means such as volume rendering are also investigated. Finally, a basic layer-based manufacturing framework is introduced to evaluate the fabrication of heterogeneous solids represented in MMa-Rep and directions for future research, aiming essentially at the impact of different interpolation strategies on manufacturing, optimization and CAD are proposed.