Data dictionaries within the Crystallographic Information Framework serve to identify and describe data items which are exchanged and archived. When data files are written or read, dictionaries can play an important supplementary role of validating individual items and checking the overall conformance of the data structure. A data dictionary contains definitions for each public tag identifying a data item that may appear in a data file. These definitions are composed of a sequence of data attributes which, individually and collectively, specify the properties of an item. These attributes are considered to be the vocabulary of a STAR File dictionary definition language (DDL). This chapter describes the concepts and specification of the STAR File DDL dictionaries used to define the data items for core crystallographic, powder diffraction, modulated-structures and electron-density applications. For these applications, it discusses the purpose of data dictionaries, the structure of a DDL dictionary file, the function of DDL attributes and the specification of DDL attributes.