The rapid growth of wireless infrastructure has created a new and highly demanding market for phased arrays. While traditionally phased arrays were only used in radar and high-end communication systems, the new market aims to expand the use of phased arrays to a very large scale, from the new generations of mobile phones to Internet of Things (IoT) devices. As a result, there is considerable interest in creating the new generation of phased array systems, which arguably require higher efficiency to integrate with regular consumer electronics. Most books on phased arrays are focused on antenna aspects or applications such as radar, but they don't teach how to design an antenna array system from the ground up. The aim of this work is to provide a text that guides the reader through a step-by-step process to design modern antenna arrays while understanding the system level requirements in each stage of the process. With a plethora of supplementary material which includes Matlab programs and electromagnetic models with commercial software, this is a complete guide to designing, building, and testing antenna arrays.