This book comprehensively covers planar slow-wave structures (SWSs), including their methods of analysis, simulation, and fabrication. Many examples of materials and configurations compatible with printed-circuit fabrication or microfabrication are incorporated. The recognition and demonstration of the potential of planar SWSs for application in traveling wave tubes (TWTs) is presented through detailed examples and case studies. The planar SWSs have a high potential to significantly reduce manufacturing costs and enhance the performance of TWTs, facilitating their adoption in emerging systems such as 5G/6G communication systems, automotive radars, and security scanners. Part of Series in Electromagnetics and Metamaterials.