A method for fabricating slow-wave structures, including electromagnetic meta-material structures, for high-power slow-wave vacuum electronic devices operating in millimeter-wavelength (30 GHz-300 GHz) and terahertz-frequency (300 GHz and beyond) bands of electromagnetic spectrum. The method includes: loading a digital three dimensional model of a slow-wave structure in a memory of a 3D printer, the loaded digital three dimensional model having data therein representative of the slow-wave structure to be fabricated by the 3D printer; loading metal powder material into the 3D printer; and operating the 3D printer to melt the metal powder material in accordance with the loaded three dimensional model of the slow-wave structure and then to solidify the melted layer of the metal powder material to fabricate the slow-wave structure layer by layer.