What they do

The administration of the ISBN system within a registration group is the responsibility of the ISBN registration agency. The ISBN registration agency may operate on a national, regional, linguistic, or other reasonable division of responsibility according to the basis of the group allocation. 

Within a group there may be several national agencies (e.g., group identifier 978-0 and 978-1 have separate agencies in Australia, English-speaking Canada, the United Kingdom, the United States and so on). 

Publishers should apply for ISBN from the ISBN registration agency that operates where they are based. The ISBN registration agency will provide publishers with all the forms and guidance that they need in order to receive ISBNs. Some ISBN registration agencies charge a fee for ISBN assignment, but you must still apply to the agency that operates where you live.