Crystallographic data may be archived or transferred in CIF data files, which associated specific data names with data values that are normally constrained to be of a certain type (integer, floating-point, text \textit{etc.}) and may have limited values (numeric ranges or specified text strings). The Crystallographic Information Framework provides compilations of recognized data names and sets of attributes that define the constraints applied to their associated values. These compilations are called dictionaries. The formalism that is used to tabulate the data names and their permitted attributes is called a dictionary definition language (DDL). Within the Crystallographic Information Framework, these dictionaries are constructed according to the same syntax and organizational rules as CIF data files, allowing a common parser to ingest and use both data files and the dictionaries that hold the key to their interpretation. This chapter describes in detail the two DDL formalisms currently used in CIF dictionaries, known as DDLm and DDL2. Both describe a relational data model used to construct and interpret CIF data files. DDLm is based on a generic extensible model; DDL2 conforms very closely to a data definition paradigm used in relational databases. Each formalism is self-referential: the definitional attributes within a DDLm dictionary are defined using DDLm, and those within a DDL2 dictionary using DDL2. The use of such self-referential data description schemes allows the consistency and relational integrity of the data model to be independently verified.