Computed tomography (CT) is a prime example of modern technologies and has found wide applications in medical imaging, industrial nondestructive evaluation, microtomography, and other areas. In this article, we focus on x‐ray CT. First, we review the development of CT, review its physical and mathematical principles, and explain the imaging geometries and systems. Then, we discuss the reconstruction algorithms, presenting both the key ideas and the mathematical details, which reflect the advancement in the field over recent years. Finally, we report future perspectives.