Many scientific applications work with structured data. HDF5 and netCDF are two well-known file formats for storing numerical data in self-describing fashion. Historically, libraries already existed to do I/O to files in these specialized formats, and as scientific applications began to incorporate parallel computation, there was a need for high level I/O libraries with parallel I/O capabilities that could interact with these established formats. This topic describes these established formats and how high level libraries are incorporated into the parallel software stack.