HDF5 is a file type designed for large volumes of data organized in a hierarchical structure. It is well suited for high-speed image data capture in crystallographic applications. NeXus is a standardized system for describing the organization of data within a HDF5 file, used in neutron, X-ray and muon science. NXmx is a NeXus definition set that supplies an ontology for macromolecular crystallography. This chapter describes the NeXus approach and gives an account of its base classes. Efforts have been made to align the NXmx ontology with the CIF ontology, especially with the image data description and experimental metadata described by imgCIF. A summary concordance between imgCIF data names and corresponding NeXus attributes is included.