The document describes HDF-EOS5, an extension of HDF used by NASA for Earth science data. HDF-EOS5 is based on HDF5 and contains standardized structures for gridded, swath, point, and zonal average data. It provides a library for reading, writing, and manipulating these data structures and their associated metadata. The library contains functions prefixed with "HE5_" for accessing, defining, input/output, inquiry, and subsetting HDF-EOS5 data.