The notion of Zero Trust Architecture (ZTA) has been introduced as a fine-grained defense approach. It assumes that no entities outside and inside the protected system can be trusted and, therefore, requires articulated and high-coverage deployment of security controls. However, ZTA is a complex notion that does not have a single design solution; rather, it consists of numerous interconnected concepts and processes that need to be assessed prior to deciding on a solution. In this monograph, we cover the principles and architectural foundations of ZTA, basically following the guidelines by NIST, and provide a detailed analysis of ZT architectures proposed by research and industry. The monograph also describes an approach for the automatic generation of ZT policies based on application communication requirements, network topology, and organizational information. This approach was designed to meet a critical need of ZTA, that is, the generation and implementation of a large number of fine-grained policies. Finally, the monograph discusses several research directions, including the incorporation of threat intelligence into ZT networks and the use of large language models (LLMs).