The Crystallographic Information File (CIF) provides a concrete syntax for an interoperable data exchange mechanism, the Crystallographic Information Framework (also CIF). A CIF file contains specific data items relevant to crystallographic structure determinations and descriptions in the form of numbers and text, which are represented as Unicode character strings. CIF is intended as a general, flexible and extensible free-format file suitable for the transmission, storage and publication of crystallographic data. Since 1990, it has been recommended by the International Union of Crystallography as the preferred data structure for the exchange of data. Two significant variants of the syntax features occur. This chapter provides a detailed description of both variants and discusses conventions for associating meaning or semantic content to included data. Particular care is taken to describe how the file format handles issues of portability and archival integrity. The chapter includes the complete official specification, and formal grammars are included in extended Backus–Naur form.