Interlining, also known as interline ticketing and interline booking, is a voluntary commercial agreement between individual airlines to handle passengers traveling on itineraries that require multiple flights on multiple airlines. Such agreements allow passengers to change from one flight on one airline to another flight on another airline without having to gather their bags or check-in again. Airlines can also promise free rebooking if the connection is lost due to a delay.

Interlining agreements differ from codeshare agreements in that codeshare agreements usually refers to numbering a flight with the airline's code (abbreviation) even though the flight is operated by another airline. However, codeshare relationships can affect whether an interline ticket (or e-ticket) can be issued, since both the codeshare marketing carrier and codeshare operating carrier must have interline agreements with all other carriers in the itinerary to allow a single ticket to be issued.

Interline agreements are directional. For example, it may be possible for American Airlines to issue the ticket on an American-United itinerary, but United might not be able to be the issuer on the same itinerary. Such a one-directional interline agreement is called a unilateral interline. The airlines may also agree to enter a bilateral interline agreement, where each airline can issue the ticket on the other airline.

Previously, only large network carriers such as United Airlines and Lufthansa would have electronic ticket interline agreements, but the IATA mandate to eliminate paper tickets at the end of 2007 has changed this by forcing smaller carriers to implement electronic ticketing.

Smaller legacy carriers commonly have interline agreements with large network carriers that fly into their markets. Most newer low-cost airlines that only sell directly to consumers (and not through agencies or Global distribution systems) do not support interlining at all.

If no interline ticketing agreement exists, then two separate tickets will need to be issued, and passengers will have to retrieve their bags and carry it to the connecting airline for check-in. Itineraries with interline connections such as this are riskier for travelers, since the second carrier may be unaware of delays or issues with the incoming flight and is less likely to permit a rebooking without cost if the connection is lost. There can also be a problem if luggage is lost and the traveler wants it to be sent to them later.

Most online travel agencies will only display itineraries that can be ticketed on one of their booking systems. However, some ticket websites will sometimes display un-ticketable interline itineraries. Examples could be found previously on routes to Mexico involving the now defunct Aero California, or may be currently found on routes to Indonesia involving Lion Air. These are often displayed as "contact airline to buy".