This chapter presents a comprehensive analysis of the applications of a low-cost version of additive manufacturing (AM). The technique called Fused Filament Fabrication (FFF), which makes use of plastic as raw material, is explained in the context of its applications to the microwave waveguide engineering field. The main advantages of this technology include the promptness to print models, the variety of feasible geometries, and specially the reduced cost. The main limitations are also explained. Two important applications are considered: (1) rapid prototyping of complex devices and (2) manufacturing of fully functional devices. The former is relevant to get a more realistic perspective of the actual geometry in computer-aided designs, as shown in several examples. It also helps to forecast possible issues in the fabrication process that the computer sometimes fails to detect at the design stage. In the latter case (2), the subsequent and necessary metallization of plastic devices is also addressed. Several examples of state-of-the-art passive waveguide devices are presented, including waveguide filters, a diplexer, a branch-line coupler, a load or horn antennas, which have been printed, metallized, and measured. The results show the potential of three-dimensional (3D) printing and provide a different insight into this innovative technology.