Gas–solid fluidized beds have historically been employed in a variety of fields owing to the excellent mixing they provide, which can enhance chemical reaction rates and make the control of the reactor temperature easier than other technologies. Due to this wide application, heuristic knowledge of their functioning has been accumulating over the years. This knowledge, however, is not always backed by a deep understanding of the physical phenomena occurring in such systems. While this heuristic knowledge is sufficient to operate fluidized beds, operation optimization and scale-up are much harder to perform. A range of diagnostic techniques have been applied over the years to draw information about the inner workings of fluidized beds. Among these, x-ray imaging techniques, especially x-ray digital radiography and x-ray computed tomography, stand out for the kind and quality of information they can provide. Their high penetrating power enables visualization of phenomena taking place in the bulk of a fluidized bed, without disturbing the bed hydrodynamics. Furthermore, x rays are generated by a source that can be switched off, making them inherently safer than other imaging techniques relying on radioactive sources, such as γ-ray computed tomography. This work gives an overview of the techniques themselves, of the quantities they can measure, and of some modern applications of gas–solid fluidized beds they have been applied to, such as waste treatment and thermochemical conversion of biomass. Overall, x-ray digital radiography and x-ray computed tomography are better suited for process understanding than for process monitoring and are extremely useful in the study of voidage distribution and macro structures, such as bubbles and jets.