The use of X-ray flow visualization has brought a powerful new tool to the study of multiphase flows. Penetrating radiation can probe the spatial concentration of the different phases without the refraction, diffraction, or multiple scattering that usually produce image artifacts or reduce the signal-to-noise ratio below reliable values in optical visualization of multiphase flows; hence, X-ray visualization enables research into the three-dimensional (3D) structure of multiphase flows characterized by complex interfaces. With the commoditization of X-ray laboratory sources and wider access to synchrotron beam time for fluid mechanics, this novel imaging technique has shed light onto many multiphase flows of industrial and environmental interest under realistic 3D configurations and at realistic operating conditions (high Reynolds numbers and high volume fractions) that had defied study for decades. We present a broad survey of the most commonly studied multiphase flows (e.g., sprays, fluidized beds, bubble columns) in order to highlight the progress X-ray imaging has made in understanding the internal structure and multiphase coupling of these flows, and we discuss the potential of advanced tomography and time-resolved and particle tracking radiography for further study of multiphase flows.