This work focuses on numerical calculations of metallic photonic band-gap structures. Photonic crystals are man-made structures that reflect electromagnetic waves whose frequency value falls within the photonic crystal band-gap. Metallic photonic crystals have been used due to certain advantages over dielectric crystals, which can be relevant for guiding THz radiations. The THz range is localised between microwave and optical frequency regions. THz frequencies have been studied for possible applications in many areas such as, imaging, security, medical, material characterization, spectroscopy to name a few. Metallic photonic crystals are used for various THz waveguides’ designs, with the aim of sustaining high power THz transmission. The transmission properties as well as dispersion relations of metallic photonic crystal are investigated by means of Finite Element Method. Throughout the study, FEM results are often compared to some other methods in order to validate the calculation steps in our modelling process and to assess the boundary conditions. In the simulations frequency dependency and losses have been taken into account. Dispersion diagrams and guided modes have been studied to achieve further understanding on the transmission characteristics of metallic photonic crystal waveguides. Excellent agreement has been obtained from the comparison of dispersion diagrams and the transmission spectra of waveguides. Special care has been paid to achieve optimized design parameters that give wide bandwidths with high transmission levels. Other passive components have also been studied such as filters, bends and splitters. In order to improve their transmission characteristics, several designs have been investigated. Consistently high transmission levels have been achieved with our waveguides over a wide range of THz frequencies. It was possible to provide innovative designs for the bend and linear waveguides. The transmission spectra of these waveguides have been analysed and a deep understanding of the metallic photonic crystal waveguide has been achieved.