Filaments, pellets, and powders are used as feedstock. Filament-based deposition is FDM, while fused pellet modeling uses pellets. Powder-based deposition is powder melt extrusion and composite extrusion modeling. Filaments are widely used feedstock, and almost all commercial systems use them, while few systems use pellets. The advantage of a filament is that it works not only as a feedstock but also as a machine part of an FDM system, because a moving filament acts as a piston to push molten filaments through a nozzle. A filament brings simplicity to AM system development. This is one of the reasons why filament-based systems are in abundance. But if the filament is to act as a piston, it must possess more properties than required by a feedstock that needs to act only as a source of materials. This is the disadvantage, which demands extra effort in its development and places extra conditions on a material to be suitable to become a filament.