There has been a rapid growth of terrestrial and satellite communications in the last few decades of the 20th century. This has resulted in a heavy congestion of low microwave bands and has been a major driving force for exploring the upper microwave and millimeter-wave frequencies. One of the main requirements for a successful shift to the new frequency spectrum is the availability of high power solid-state transmitters. Solid-state devices such as diodes or transistors have been able to meet such demands when their output signals are combined using space-level power combining methods that avoid conduction losses, which become pronounced at millimeter wave frequencies. In this thesis, theoretical and experimental investigations are carried out into the spatial power combiners (SPCs) which employ active planar arrays formed by transistor amplifiers whose input and output ports are equipped with planar radiating elements. The SPC structures include the reflection-type combiner using the tile configuration of planar array and the transmission-type combiner using tile or tray configurations of planar arrays. The frequency bands chosen for the designing and testing of prototypes are X- and Ku-band. The first stage of the investigation concerns the 10 GHz reflection-type power combiner structure formed by a phased planar microstrip reflectarray (MRA) of 37-element dual-feed aperture coupled microstrip patch antennas equipped with open-circuit stubs as phasing components. The experimental tests reveal poor radiation performance and hence poor power combining efficiency of this structure. These results indicated the need for theoretical investigations into the operation of this type of SPC. The study of the unit cell of this power combiner reveals that the phase of an open-circuit stub does not increase linearly as a function of the stub length and its range is limited to less than (about is required for proper functioning). This finding, forms the basis for extending the investigations into alternative phasing mechanisms of a MRA which would offer a phasing range exceeding . A phasing mechanism exploiting variable size stacked patches is chosen. In order to accurately determine the phasing of the reflected wave, a theory based on an equivalent unit cell waveguide approach (WGA) is proposed and developed. The proposed theory is computationally efficient and is proven to be accurate compared with benchmark results published by other researchers. Following the verification, an offset feed 161-element two-layer printed MRA prototype with patches of variable size is designed and developed for operation in Ku-band. The test results aim at verifying the validity of applying a unit cell WGA to designing passive and active MRAs. The next investigations, which are presented in the thesis concern increasing operational bandwidth of the transmission-type SPC in tile configuration. The designs presented so far in the open literatures were based on edge-feed microstrip patch antennas as radiating elements of individual active stages and featured a narrow-band performance. In order to overcome this shortcoming, stacked patch (SP) microstrip antennas as receiving and transmitting elements in an active transmitarray (TXA) are proposed. For the aim of testing the proposed concept, a 16-element SP TXA is designed for operation in X-band. Two identical hard horn antennas with an approximately uniform field across the aperture for signal launching and collecting complete the design and development of this space-level power combiner. The performance of the developed device is assessed experimentally and an increased operational bandwidth is demonstrated. The final structure being investigated in the thesis project is the transmission-type SPC in tray configuration. This power combining structure employs a travelling wave antenna of uniplanar quasi-Yagi type as a radiating element to achieve broad-band operation. The investigated SPC is formed by seven trays of uniplanar quasi-Yagi antenna. In order to achieve uniform and in-phase excitation of individual trays, which is required to obtain high power combining efficiency, hard horn antennas and Schiffman phase shifters are employed in the design of this space-level combiner. The proposed device is developed and its performance is assessed through experiments. The work performed as part of this Ph.D. thesis project has resulted in 5 journal papers and 11 refereed conference papers. This acceptance rate supports the claim of the originality and significance of the research undertaken as part of the thesis project.