A library catalogue holds one or more record for every bibliographic item in order to facilitate access to the library's collection. But how are bibliographic records created and processed in order to generate these catalogue entries? In other words, how would a computer program recognize the different components of a bibliographic record and process them appropriately? There are two sets of problems here: first, we need to decide which data elements to store, and then we should decide how each data element is to be processed. Bibliographic data formats facilitate the creation, management and exchange of bibliographic data by prescribing the various data elements necessary to describe a bibliographic item, and also by providing guidelines for the processing of data stored for each data element. Over the years several bibliographic data formats have been created, and these all conform to the international standard ISO 2709: Format for Bibliographic Information Interchange. Of these, MARC is the most common – MARC 21 is the most widely used format in the world. This chapter begins with a discussion of the nature and characteristics of bibliographic formats, followed by a description of the structure of bibliographic data interchange format as given in the international standard ISO 2709. It then goes on to outline the various components and features of MARC 21, and of two other bibliographic formats that are used in different parts of the world – UNIMARC (UNIversal MARC) and CCF (common communications format).