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The goal of this dissertation is to design novel metamaterials for High Power Microwave devices. Metamaterials (MTM) are artificial periodic structures, which support reversed Cherenkov radiation and backward wave propagation. Microwave sources transform the kinetic energy of an electron beam into microwaves through the interaction of the electrons with a periodic slow wave structures (SWS). Here, a metamaterial (MTM) waveguide is proposed for use in a microwave oscillator instead of a SWS. The interaction of a multi-beam cathode with a set of MTM structures inside a cylindrical waveguide is studied and analyzed. Group theory is used to design a metamaterial slow wave structure that uses structures with dissimilar unit cell dimensions to tune the negative permittivity and negative permeability to overlap at the same frequency. The structure is arranged azimuthally such that it can be excited using a multi-beam emitter. Using a multi-beam has some advantages. It increases the interaction with the metamaterial and therefore it enhances the output peak power (105 MW) of an BWO. The second part of dissertation is dedicated to the design of a power combiner/splitter for THz applications. Most dangerous explosive materials, both toxic and radioactive, contain nitrogen salts with resonant absorption lines in the frequency range 0.3-10 THz. Therefore, there has been a growing interest in remotely detecting such materials by observing the spectrum of reflected signals when the suspicious material is interrogated by THz radiation. Practical portable THz sources available today generate only 20–40 mW output power. This power level is too low to interrogate suspicious material from a safe distance, especially if the material is concealed. Hence, there is a need for sources that can provide greater power in the THz spectrum. Generating and extracting high output power from THz sources is complicated and inefficient. The efficiency of vacuum electronic microwave sources is very low when scaled to the THz range and THz sources based on scaling down semiconductor laser sources have low efficiency as well, resulting in the well-known “THz gap.” The reason for such low efficiencies for both source types is material losses in the THz band. In this work an efficient power combiner is described that not only combines the THz power output from several sources, but can also form a Gaussian wave beam output. A minimum conversion efficiency of 89% with co-phased inputs in a lossy copper power combiner and maximum efficiency of 100% in a Perfect Electric Conductor (PEC)-made power combiner were achieved in simulations. Also, it is shown that the TE01 output mode is a reasonable option for THz applications since conductive losses decrease for this mode as frequency increases.