The essential code for descriptive cataloguing is the Anglo-American Cataloguing Rules (AACR), first developed in 1967 and updated regularly until 2005. The revisions and updates of the standard are referred to as AACR2. RDA is a new standard for descriptive cataloguing aimed to be a replacement for AACR2. This chapter first presents the evolution of codes from Panizzi’s Rules to RDA and then focuses on the structure of AACR2. The main aim of the text is to provide an overview of the structural parts of AACR2 and the process of describing resources with the standard.