You go through security again on a layover only if you leave the secure side of the airport, or if the airport you are connecting through screens all transfer passengers as a matter of policy. Stay airside and, at most airports, you walk straight to your next gate. The whole question comes down to knowing which of those situations your connection falls into before you fly.
Airports are split into two zones: the landside area anyone can enter, and the secure airside area past the checkpoints. Security screening exists to control the border between the two. So the question is never really about layovers at all. It is about whether anything on your connection pushes you back to the landside zone.
Three things do: arriving into the United States from another country, connecting on separate tickets, and changing terminals or airports where no airside route exists. One more thing forces screening even if you never leave airside: a handful of airports, most famously London Heathrow, screen every transfer passenger on principle.
| Your connection | Security again? |
|---|---|
| Domestic to domestic in the US, same airport | No. You stay airside and walk to your gate. |
| Arriving into the US from abroad, any onward flight | Yes. After immigration and customs you are landside, so you clear TSA again. |
| Arriving into the US from a preclearance airport | No. You cleared US formalities before departure and arrive as a domestic passenger. |
| Connecting inside the Schengen area | Usually no. Airside transfers normally skip screening. |
| Arriving in Europe from outside the EU, connecting on | Usually yes. Most hubs run transfer screening for arrivals from outside the EU system. |
| Any connection at London Heathrow | Yes. Heathrow screens all transfer passengers at the flight connections checkpoints. |
| Separate tickets | Yes. Collecting and rechecking a bag puts you landside, so you re-enter security from scratch. |
| Changing terminals with no airside route, or changing airports | Yes. Landside transit always ends at a checkpoint. |
If your connection is not in the table, the rule still answers it: work out whether you will end up landside. When in doubt, ask the crew before landing. They make this connection every week and will know.
Most large airports outside the US have a sterile international transit corridor, so passengers passing through never formally enter the country. US airports do not. Every arriving international passenger enters the United States at the first airport, which means immigration, collecting your checked bag, customs, and then the exit door into the landside hall. Your onward flight is now, administratively, a fresh departure: bag on the recheck belt, and you through the TSA checkpoint again.
This is why US connections on international arrivals need more time than the same connection would domestically. The checkpoint queue you cleared at your origin is waiting for you again in the middle of your trip, stacked on top of the immigration queue. Our guide on how long a layover should be puts numbers on it, but the short version is that two hours is a sensible floor for an international-to-domestic US connection, and less than 90 minutes is a gamble.
The exception is preclearance. Fly to the US from Dublin, Shannon, Abu Dhabi, and the major Canadian and Caribbean preclearance airports, and you complete US immigration and customs before boarding. You land as a domestic arrival, stay airside, and connect without seeing another checkpoint.
Inside the Schengen area, an airside connection normally involves no security screening at all. You may cross a passport line when your route enters or leaves Schengen, but passport control and security screening are separate processes, and clearing one does not mean queueing for the other.
Arrive in Europe from outside the EU security system, and most hubs route you through a transfer screening point before you reach your departure gate. The practical reason is liquids: your origin airport may apply different rules, so the connecting airport re-checks the bag you are carrying. A short list of countries has screening recognised by the EU, so some transfers skip this, but the safe assumption is that you will be screened, and that your cabin bag needs to pass a fresh liquids check mid-trip.
The UK is stricter in one specific way. Heathrow screens every transfer passenger, whatever the origin, at dedicated flight connections checkpoints. Budget for it exactly as you would a normal security queue, and note that the same liquids limits apply.
One more case catches people on flights to the US from anywhere in the world: gate screening. Many airports run an additional document check and random bag search at the gate for US-bound departures. It is quick, but it is one more reason not to arrive at the gate at the last minute.
Re-screening is where layover duty free goes to die. A bottle bought airside at your origin is over the liquid limit the moment you have to pass another checkpoint, and whether it survives depends on how it is packed.
In Europe and most transfer hubs, duty free liquids survive screening if they are in the sealed tamper-evident bag the shop packed them in, with the receipt visible inside, bought that day. Leave the seal intact until your final destination.
In the United States the sealed bag does not help at the TSA checkpoint, but you do not need it to. You are standing at the customs exit with your checked suitcase anyway, so open it and put the bottle inside before you drop the bag on the recheck belt. Crew do exactly this. The only passengers who lose duty free in the US are the ones who left it in their cabin bag.
If your itinerary has a re-screening connection and you cannot check the liquid, buy at the last airport before your final destination instead.
Transfer checkpoints are usually faster than the main landside queue, because they only process connecting passengers. Fifteen to thirty minutes is typical; a US international-to-domestic connection is the slow exception because immigration and bag recheck come first. Build your buffer around the slowest link, not the average, because missing the connection costs far more time than the buffer ever would. On a single ticket the airline rebooks you if the queues win; on separate tickets the risk is yours.
Two small habits make the second screening painless. Keep your liquids bag and electronics near the top of your cabin bag rather than burying them after the first checkpoint, and keep your pockets empty from the start. People who fly for a living pack once, in checkpoint order, and never repack mid-trip.
And if the re-screening question makes you wonder whether you can use the layover to see the city instead: that is a different calculation, covered in can you leave the airport during a layover. Leaving always means clearing security again on the way back in, so the answer there depends on the same clock.
CrewVIP is built by and for airline crew, with discounts in 6,000+ cities and layover guides written by people who work these connections every week.
Not on a domestic US connection: you stay airside and walk to your gate. Arriving from another country, yes: after immigration and customs you are landside, and you clear TSA again for the onward flight.
Yes. The customs exit leads to the public landside area, so the onward flight starts with a normal TSA checkpoint. Drop your checked bag on the recheck belt first, then head to security.
Connecting inside the Schengen area airside, usually not. Arriving from outside the EU security system, most hubs screen transfer passengers before the departure gate, mainly to re-check liquids.
Yes. Heathrow screens all transfer passengers at its flight connections checkpoints regardless of where they arrived from, with standard liquid limits applied.
Yes. You collect your bag, exit landside, check in with the second airline, and pass security from the start. Treat the connection as a full departure and budget time accordingly.
In Europe, yes, if they stay in the sealed tamper-evident bag with the receipt, unopened. In the US, put them in your checked suitcase at the customs recheck belt; the sealed bag alone will not get them through TSA.