In the last decade, nanomaterials have gained considerable attention and interest in the development of new and efficient molecular probes for medical diagnosis and imaging. Compared to traditional contrast agents used from the 70s, this comes from the new possibilities offered by the increased half-life of nanosystems in blood stream, as well as by the specific accumulation in organ of lesions through passive or active targeting mechanisms. Heavy metal or iodinated-loaded nanoparticles are excellent absorbers of X-rays and can offer excellent improvement in medical diagnosis and X-ray imaging. This review aims to propose an accurate state-of-the-art of the emerging applications of nanotechnology in X-ray imaging. Likewise we will discuss and compare all the solutions proposed, and the impact of their composition, formulation methods, and physicochemical properties on their applications, efficiency, and potential industrial scaling-up.