This paper describes a file definition facility (FDF) for defining files as graph structures. The structure of the file is explicitly declared in the file definition. Primitive functions (from graph theory), operators, and the format of the definition statements are given. The combination of functions and operators appear as directives to the programming system for structuring files.Several simple examples are given to illustrate the use of the FDF. The data organization for the implementation of this facility is described in detail. Problems of considerable importance that are treated are: (1) garbage collection, (2) template construction, and (3) runtime address calculation. The external definitions are represented internally by descriptors. The format of the descriptors is given and a discussion of the items in the descriptors is presented.