**6. Conclusions**

This chapter provides a wide overview of metal 3D-printing applied to the design of various types of waveguide components, antennas, and versatile periodic structures. It can be concluded that the use of 3D-printing can bring multiple relevant advantages when a set of guidelines are followed from early steps in the conception of the RF devices. This chapter opens wide perspectives for the conception of new disruptive RF devices through 3D-printing. The future trend concerns the development of 3D topologies with enhanced performance, complex functionality, and many degrees of freedom. New synergies are expected in RF design combined with powerful optimization techniques that are capable of managing such amount of geometrical parameters. Finally, new theoretical models are expected to appear, providing valuable physical insight on the phenomena underlying the operation of this new 3D structures.
