5G millimeter-wave (mm-wave) applications have several challenges, such as detection accuracy, interference from other channels, high path loss, covering several users in a dense area, and vulnerability to the environmental conditions. Several antenna technologies were proposed in this dissertation to address some of the above-mentioned challenges in 5G applications with a focus on wireless networks and vehicle to everything (V2X) communications. Leaky-wave antennas (LWAs) are suitable candidates for 5G mm-wave applications due to their beam-scanning capability, compactness, low cost, and ease of fabrication. Substrate integrated waveguide (SIW) and half-mode substrate integrated waveguide (HMSIW) are suitable candidates for realizing LWAs because of their low-profile and integration capability. Several design approaches were introduced in this dissertation to improve the performance of SIW/HMSIW LWAs in 5G mm-wave applications. Some of the proposed antennas have a relatively wide beam-scanning range, suitable for radar systems and V2X communications, while others have small beam-squint, suitable for point-to-point communications and seeker antennas. Tapering the side aperture of an HMSIW and the feed transition of a SIW resulted in side-lobe level (SLL) reduction to enhance the detection accuracy and reduce sensitivity to interference. Applying the proposed methods reduced the SLL of an HMSIW LWA and a SIW LWA to -11.2 dB and -11.4 dB, respectively, in the mm-wave frequency band. Furthermore, embedding cavities into a compact low temperature co-fired ceramics (LTCC) antenna enhanced the gain to 7.6 dBi at 28.5 GHz. Implementing different types of reconfigurable structures resulted in electronic beam-scanning, which is the most suitable approach to provide coverage for several users in dense areas due to its ease of implementation. Each of the proposed reconfigurable antennas posed different scanning ranges. One example used varactor diodes for tuning the antenna and achieved 30⁰ of beam-scanning range by varying the varactor diode's capacitance in the range of 200−500fF. Moreover, the bias circuitry was integrated into the RF ground in a few designs to miniaturize the reconfigurable antenna. The compactness, beam-scanning capability, low SLL, medium to high gain, and low fabrication cost are among the features that make the proposed antennas suitable candidates for 5G mm-wave applications.