Didn’t print a copy of your ESTA registration number? You can now retrieve it.
If travel under ESTA has been approved, you are not required to present a written copy of your regsitration number when traveling. The carrier is able to scan your passport for confirmation of ESTA approval at the time you check-in.
However, the ESTA website has recently been updated and travelers who did not keep their reference number are able to retrive it. Please visit https://esta.cbp.dhs.gov for further information.