Book excerpt: Recently, there has been increasing interest and rapid growth in millimeter wave (MMW) antennas and devices for use in diverse applications, services and technologies such as short-range communication, future mm-wave mobile communication for the fifth generation (5G) cellular networks, and sensor and imaging systems. Due to the corresponding smaller wavelength, mm-wave frequencies offer the advantage of physically smaller antennas and circuits as well as the availability of much wider bandwidth compared to microwave frequencies. It is important to design millimeter wave antennas with high gain characteristics due to their high sensitivity towards atmospheric absorption losses. Moreover, millimeter wave antennas can have wide bandwidth and are suitable for applications in large frequency range. In this thesis, planar antennas are designed using substrate integrated waveguide (SIW) technology to have low losses, high quality factor, and low fabrication cost. Firstly, an antipodal fermi linear tapered slot antenna (AFLSTA) with sine corrugations at the side edges at 32.5 GHz is presented, which has a wide impedance bandwidth greater than 30 %, in order to support the high data rate channels. This antenna has a high gain of 12.6 dB and low side lobe levels (better than - 17 dB) in both E and H planes. This antenna is studied and analyzed in array and beamforming configurations to meet requirements of millimeter wave applications. In order to obtain high gain and narrow beamwidth pattern, a 1 × 8 AFLTSA array using SIW power divider network is presented. The design characteristics of the power divider network are presented in this thesis, which help in calculating the performance characteristics of this array structure. This array has an acceptable bandwidth of 14.7 % (30-35 GHz) with high gain of 20.4 dB and 8.35° 3 dB beamwidth. The side lobe levels are also improved using this SIW power divider network and are lower than -25 dB in E-plane and -15 dB in H-plane respectively. This antenna has a radiation efficiency greater than 93% over the whole bandwidth. The second research theme is beamforming of AFLTSA antenna. This beamforming is performed using multi-beam antenna concept in which the beam is rotated with a help of compact beamforming network and excitation from different input ports. The design methodology for 2 × 2 and 4 × 4 subarray beamforming networks is presented along with their current distributions illustrating the beamforming process. These subarrays possess wide impedance bandwidth between 29-36 GHz. Moreover, these subarrays are able to achieve gain between 12-15 dB with narrow beamwidth reaching till 11°. All the results along with the numerical data is presented in this thesis. This antenna is suitable candidate for millimeter wave wireless communications and imaging systems.