13.1. Introduction. The present chapter describes the Harmonized Commodity Description and Coding System (HS), which is recommended by IMTS 2010 (para. 3.11) for the collection, compilation and dissemination of international merchandise trade statistics at the most detailed level. It is based on chapter III of IMTS 2010, on commodity classifications. This chapter provides an overview of the HS and details of the latest (fifth) edition of HS which entered into force on 1 January 2012 (HS12), discusses application of HS for coding traded goods as well as some particular aspects of quality assurance related to the classification of goods (quality assurance in general is covered in chap. IX), and describes country experiences in the use of HS for data dissemination and analytical purposes. Understanding HS by trade data compilers is necessary for a better understanding of the classification decisions made by national customs authorities, for setting up appropriate validation and editing checks, for communicating more effectively with both customs authorities and the user community, and for developing the ability to assist in the formulation of proposals for future revisions of HS. Other international classifications relevant primarily to the dissemination and analysis of international merchandise trade statistics are described in chapter XXVII.