Clusters and nanocrystals constitute intermediates between molecules and condensed matter. Due to their finite size, clusters have a wide spectrum of applications ranging from building blocks for novel materials to model systems for fundamental investigations about light-matter interactions. Short-wavelength radiation from synchrotron radiation sources and free-electron lasers allows the detailed investigation of their geometric, electronic, and magnetic structure as well as dynamical processes. Conversely, clusters can serve as idealized sample systems for the development of new experimental techniques and pioneering experiments with novel x-ray sources. The chapter starts with a brief introduction to cluster physics, followed by a comprehensive overview of research performed at synchrotron light sources on van der Waals, metal, and semiconductor clusters. With the advent of short-wavelength free-electron lasers, a new research field in the x-ray peak intensity regime has opened. Experiments on single clusters, such as x-ray imaging and tracing ultrafast dynamics, now become possible.