This chapter provides details of function libraries for programmers developing CIF applications or adapting existing applications to work with CIF. Extensive documentation is provided for the Fortran library CIFtbx, which provides a comprehensive set of input/output routines for handling CIF data, support for checking data-name aliases, and limited type checking if suitable dictionaries are available; and for CBFlib, an ANSI C library for manipulating image data. Other libraries suitable for different purposes are discussed in less detail, including Perl (COD::CIF::Parser), Python (PyCIFRW) and Julia (CrystalInforFramework.jl) implementations for general CIF applications, C++ (CIFPARSE) and Python ( mmLib py-mmcif) libraries for mmCIF data, the Julia package ImgCIFHandler.jl for image data, and the Python NeXpy library for images and experimental metadata in HDF5 files. There are discussions on how to choose the most appropriate library for a particular purpose, and a survey of CIF parsers, both standalone tools and as incorporated in some of these lilbraries.