This thesis presents studies of spatial waveguide power combiners for communication systems. The previous work on spatial power combiners is reviewed thoroughly. After analyzing the pros and cons of all the existing structures, two types of spatial power combiners are proposed. The first one is a new transitional structure from a rectangular waveguide to multiple microstrip lines for high-efficiency power combining applications. The fields of microstrip lines in a shielded rectangular waveguide are obtained by solving a set of homogeneous simultaneous equations. Mode-matching techniques are successfully implemented to analyze the N-furcated rectangular waveguide, the junction between rectangular waveguide and multiple microstrip lines, and the junction between multiple microstrip lines in one rectangular waveguide to multiple microstrip lines in multiple shielding waveguides. Calculated results are verified by the state-of-art field simulation software and excellent agreements are observed. The designed power combiner is fabricated and very good performance has been achieved. The second one is an eight-way radial power combiner. Eight suspended substrate stripline (SSS) peripheral ports are used in the design to minimize the loss and realize a compact design. The field in a SSS is solved using a mode-matching analysis. Good experimental results are presented, which show very good return loss and insertion loss performance. Conclusions and recommendation for future works are given at the end of the thesis.