This book illustrates the exciting possibilities being opened up by X-ray computed tomography (CT) to follow the behavior of materials under conditions as close as possible to those encountered during their manufacture or in operation.The scientific chapters selected for this book describe results obtained using synchrotron or laboratory devices during in situ or ex situ experiments. They characterize microstructures across length scales ranging from tens of nanometers to a few tens of micrometers.In this collection, X-ray CT shines a light on the mechanical properties of engineering materials, such as aluminum or magnesium alloys, stainless steel, aluminum, polymer composites, or ceramic foam. In these experiments, X-ray CT is able to image and quantify the damage occurring during tensile, compression, indentation, or fatigue tests.Of course, X-ray CT can illuminate the structure and behavior of natural materials too. Here it is applied to bone or natural snow to study their mechanical behavior, as well as materials from the agri-food sector. Its versatility is exemplified by analyses of topics as diverse as the removal of olive oil from kitchen sponges by squeezing and rinsing, to the effect of temperature changes on the structure of ice cream.Some chapters focus on changes occurring over time, at different temperatures, humidity levels, pressure, as in the case of heat treatments of aluminum alloy, concrete ageing, or ceramic foaming processes.In response to the challenges of climate change, research activities on batteries have intensified. The non-destructive and three-dimensional nature of X-ray CT have made it a very valuable tool for monitoring their evolution during charge and discharge cycles, as illustrated by some of the contributions.Issues relating to the technical development of CT are also covered, for example, helical CT, diffraction contrast tomography, or the use of contrast agents. The effects induced by the exposure of materials to X-ray radiation are also discussed.Finally, aspects relating to post-processing (e.g., procedures to improve the reconstruction of samples that move during in situ tests) and data analysis (e.g., the application of digital volume correlation (DVC) the 3D analogue of digital image correlation (DIC), and the comparison between in situ experiments and finite element simulations) are covered. Taken together these studies show the art of what is possible, the ways to further enhance the existing methods, and the possibilities for the future in relation to the In situ X-ray Tomographic Study of Materials.