A comprehensive survey of Open RAN technology and its ecosystem. In Open RAN: The Definitive Guide, a team of distinguished industry leaders deliver an authoritative guide to all four principles of the Open RAN vision: openness, virtualization, intelligence, and interoperability. Written by the industry experts currently defining the specifications, building the systems, and testing and deploying the networks, the book covers O-RAN architecture, the fronthaul interface, security, cloudification, virtualization, intelligence, certification, badging, and standardization. Virtualization and cloud have unified computing and networking. This unification can be seen in all areas of wired communications. With Open RAN (O-RAN), wireless communications will be able to take advantage of the unification as well. Wireless communications are overtaking wired communications. According to some estimates, traffic from wireless and mobile devices accounts for two-thirds of all IP traffic. However, the deployment of wireless communications is costly. The cost is mainly driven by RAN. For example, substantial increase in the number of cell sites and radio types with massive MIMO (multi-input multi-output) technology required by 5G drives RAN costs even higher. O-RAN attempts to solve this problem by disaggregating and splitting the RAN, supporting standardized interfaces that can interoperate with each other. This standardization drives multivendor radio, hardware, and software deployments with key functions implemented as virtual network functions (VNFs) and cloud-native network functions (CNFs) on vendor-neutral hardware and networks implemented in modular units.