I am traveling from Milwaukee to Chicago to Newark Liberty (New Jersey) to Edinburgh. Will I have to retrieve my bag at the New Jersey airport and recheck it for the flight to Edinburgh? It is all the same airline, United.
Assuming that you've purchased a single ticket for this trip (instead of going to United and buying two separate tickets), your checked luggage will be checked through to Edinburgh, and you will not need to claim it at Newark or anywhere else along the way.
Note, however, that if you take the same route in reverse on the way home, you will need to claim your bag at Newark when you go through US Customs. After you take it through Customs, there is a baggage drop where agents will take your bag back and return it to the system; you'll find it again (barring any mishaps, of course) when you land in Milwaukee.
1I have also done it where I booked 2 separate tickets on American Airlines (SFO-ORD on one, and ORD-DUB on the other). Because they were both AA flights, I was able to check my bag from DUB all the way to SFO.– MichaelApr 28, 2017 at 18:59
2Just to be overly pedantic: even if you return via an airport other than Newark, you'll have to go through this procedure at the first airport you land at in the US. (Unless you're doing something weird like flying back via Ireland or Canada.) Apr 28, 2017 at 18:59
@MichaelC.: DUB is kind of a weird case because you clear US customs & immigration before the plane takes off. This means that passengers arriving from Dublin or Shannon don't have to reclaim their luggage to go through customs upon arrival in the US. (The same thing applies to most major Canadian airports, along with a few in the Caribbean.) If you had been coming from another international destination, you probably would have had to reclaim your checked bags. Apr 28, 2017 at 19:04
@MichaelSeifert Oneworld used to have a policy where they would check your bags through if you booked two separate tickets, but they eliminated it last year and American won't do it anymore, at least officially, if you have two separate PNRs. United supposedly will, but your mileage may vary at the checkin counter depending on the agent you get. Apr 28, 2017 at 19:09
1@MichaelSeifert Michael C was talking about a flight from SFO to DUB, not from DUB to SFO, so the presence or absence of a preclearance facility at DUB wouldn't make a difference in that case. He's just saying that when you book 2 tickets on the same airline, you can check your bag all the way through to your final destination rather than needing to re-check it at the break point between the two tickets.– reirabApr 28, 2017 at 19:10