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Abstract: Three-dimensional (3D) measurements of strain tensor distributions with subgrain resolution are now possible due to an emerging class of instrumentation: the 3D X-ray crystal microscopes. These instruments combine ultra-intense synchrotron X-ray sources and advanced X-ray optics to probe polycrystalline materials with submicron spatial resolution. By employing polychromatic X-ray beams and a virtual pinhole camera method, called differential aperture microscopy, 3D distributions of the local crystalline phase, orientation (local texture) and elastic and plastic strain tensor distributions can be measured with resolution below the grain size of most materials. The elastic strain tensor elements can typically be determined with uncertainties less than 100ppm in brittle materials. Orientations can be quantified to ∼0.01° and the local unpaired dislocation density can be measured in ductile materials. Efforts are underway to improve the spatial resolution and increase the depth probed. Because the 3D X-ray crystal microscope is a penetrating nondestructive tool, it is ideal for studies of mesoscale structural evolution in materials. The unprecedented information that can be obtained with this new class of instrumentation is certain to advance our understanding of the materials physics underpinning the behavior of polycrystalline materials. [Copyright &y& Elsevier]