The phased array is a key technology for Millimeter-Wave (mmWave) communication and sensing. The phased array consists of two or more antennas and features beamforming, beam steering, and beam tapering. With beamforming, the radiation pattern of a phased array can be narrowed to a concentrated beam compared to a single antenna. With beam steering, the phased array enables directional communications by controlling the phase difference of the individual antennas. And with beam tapering, the shape of the radiation pattern can be manipulated to suppress the power loss in the unwanted directions by altering the amplitude distribution of the individual antennas. In this thesis, fundamental features and principles of phased array antennas, including the configuration of phased array, array factor, beamforming, beam steering, grating lobes, and beam tapering, are explored and demonstrated using modeling, theoretical derivations, and MATLAB simulations.