NeXus is an international standard for the storage and exchange of neutron, x-ray, and muon experiment data. The structure of NeXus files is extremely flexible, allowing the storage of both simple data sets, such as a single data array and its axes, and highly complex data and their associated metadata, such as measurements on a multi-component instrument or numerical simulations. NeXus is built on top of the container format HDF5, and adds domain-specific rules for organizing data within HDF5 files in addition to a dictionary of well-defined domain-specific field names.