X-ray computed tomography (X-CT) has become an important tool for the nondestructive characterization of defects and mechanical properties in additive manufacturing (AM), providing accurate qualitative and quantitative analysis of internal structures and defects in fabricated components. Defects significantly influence the mechanical performance and service life of materials, whereas variations in AM process parameters, final part orientation, and raw materials can affect defect characteristics. X-CT not only effectively distinguishes critical defect zones from benign regions prior to mechanical testing but also enables in situ observation of defect initiation and evolution during loading, supporting the development of defect tolerance models. This review focuses on the applications of X-ray tomography in additive manufacturing, particularly in analysing internal defects and their impact on key mechanical properties such as static strength and fatigue behaviour. High-resolution tomographic imaging allows precise evaluation of defect type, location, and size and their impact on comprehensive mechanical properties, offering critical insights for optimizing AM processes and improving component quality and reliability.