Compact antennas with high radiation efficiency have found numerous applications in wireless and satellite communication systems in recent years. Among planar antenna structures, waveguide slot arrays have received considerable attention because of their high aperture efficiency, low cross polarization and low side lobe levels. At high frequencies, fabrication of these antennas is very expensive due to highly accurate milling machines that are required. Furthermore, because of using conventional rectangular waveguides, these antennas are bulky and are not suitable for integration with printed microwave circuits. A rectangular waveguide can be implemented within the printed circuit board substrate by drilling two periodic arrays of metalized via holes between two ground planes acting as side walls of the waveguide. The resulting device is called Substrate Integrated Waveguide (SIW) and its dominant guided mode is basically a slightly perturbed TE10 mode of the equivalent conventional dielectric filled waveguide. Recently, many waveguide based devices and components have been integrated with planar microwave circuits using this technology. Slot array antennas can also be fabricated using SIW at microwave and millimeter-wave bands using low cost PCB technology. SIW-based slot array antennas that have been reported so far consist of one layer of substrate and are fed from one end through a coplanar feed network that significantly increases the size of the antenna. Furthermore, radiation from microstrip feed lines can severely compromise the low side-lobe level of the antenna array and generate cross polarization. In order to overcome these shortcomings, in this thesis a novel two-layer SIW-based slot array antenna is introduced which employs a feed waveguide integrated within the bottom substrate of the antenna structure while the top substrate layer contains the slotted waveguide arrays. Each branch of the slot array is coupled to the feed waveguide through a slanted slot and the antenna is excited by a microstrip line printed on the back. After describing the design algorithm of slot array antennas, several arrays are designed, simulated, and three of them are also fabricated and measured. Two antennas are designed and simulated at 12.5 and 24GHz demonstrating 31dB and 27dB gain, respectively. Two planar square arrays with 6x6 and 4x4 elements and a linear array of 14 elements, all operating in 60GHz band, are also designed, simula Key Words: Waveguide slot arrays, Substrate integrated waveguide (SIW), Millimeter-wave antenna arrays, Sensitivity analysis