Ebooks: the ASIN does the job on Amazon
Amazon's catalog runs on ASINs, which every Kindle book receives automatically and free. KDP does not require an ISBN for ebooks, and most self-publishers skip it there. Some other retailers, aggregators, and library platforms do want an ebook ISBN, so authors distributing widely sometimes assign one anyway — check the requirements of each channel rather than assuming Amazon's rule applies everywhere.
Print books: required, but free if you want
A print edition cannot be published on KDP without an ISBN. The free KDP ISBN is genuinely free and instant, with two trade-offs: it is valid only on KDP, and the imprint of record shows as Independently published. Buying your own ISBN — through Bowker's myidentifiers.com in the United States, or free or paid national agencies elsewhere — keeps the same number usable at KDP, IngramSpark, and any other printer, under your own imprint name.
One ISBN per edition, not per book
The ISBN identifies a specific edition and format, not the work itself. A paperback and a hardcover of the same title carry different ISBNs; a large-print edition is a third. A cover change alone does not create a new edition — you can update a cover and keep the ISBN — but changing trim size or format does.
ISBNs and barcodes are two different things
The ISBN is the number; the barcode is its scannable EAN-13 rendering printed in the back cover's barcode zone. If your cover file has no barcode, KDP places one automatically in its reserved area. If you want control over placement — or you print anywhere that does not add one — generate the barcode from your ISBN and keep it on a light background inside the template's designated zone. The free tool below validates an ISBN-13 and draws a scalable barcode.
Sources: International ISBN Agency · Bowker — United States ISBN agency · Amazon KDP help center. Reviewed ; recheck the official page before production.