The goal of this talk is to educate HDF5 users about backward and forward compatibility issues across releases of the HDF5 Library and versions of the HDF5 file format. We will discuss changes in the file format that were done to support new HDF5 features such as object creation order, compact groups, efficient access to the variable length data, UTF-8 encoding, external links, etc., and their implications on the HDF5 Library and users’ applications.