The CTD is the internationally recognised standard format for submissions to medicines regulatory authorities. In the European Economic Area, the USA and Canada, the CTD, in its electronic format (eCTD), is mandatory for all applications for marketing approval and all subsequent related submissions. The CTD is accepted in many other countries, being mandatory for new prescription medicines in some. This module explains the rationale for the CTD and provides guidance on its structure and format and the ways in which it is used.