2021 marks the 50th anniversary of x-ray computed tomography (CT). Over the years, CT has experienced tremendous technological development, driven mainly by clinical needs but also by technology advancements in other fields. Six years after the third edition of Computed Tomography, this fourth edition captures the most recent advances in technology and clinical applications. New to this edition are descriptions of artificial intelligence, machine learning, and deep learning, and their application to image reconstruction, protocol optimization, and workflow. A new chapter covers the principles and advances in dual-energy and spectral CT. A new detector technology, the photon-counting detector, is described in detail, and its impact on CT system and clinical applications is analyzed. Many exciting developments in clinical applications, such as cardiac functional imaging and stroke management, are also covered in detail.