You don’t have to be a cryptocurrency investor to know that the vast majority of banking and finance is now conducted electronically.

From monitoring account balances online to paying for chewing gum with your phone, technology now allows the average person to go about their everyday life without ever needing bills or coins in their pockets.

But physical currency is still very much part of our society—more than $2 trillion in more than 50 billion U.S. notes was moving through the world at the end of 2020, according to the Federal Reserve.

Most of us don’t carry more than a couple hundred dollars on our person at any given time, but there are instances in which that number can be much higher. And while it is perfectly legal to carry as much money as you like, there are times when it requires extra diligence, such as at the airport.

The first thing to know is that these guidelines apply to all physical forms of money: cash, coins, foreign currency, traveler’s checks, personal checks, endorsed money orders and so forth. A bookbag stuffed with $100,000 in $20 bills and a $100,000 promissory note tucked in a wallet are treated exactly the same way.

If you’re flying domestically—your departure and arrival points are both in the United States—you are allowed by law to carry as much money as you want. However, if you are entering the U.S. from another country, you are required to declare to a U.S. Customs agent that you are carrying any amount of money over $10,000.

You also are required to fill out a FinCEN Form 105, which is used by the Financial Crimes Enforcement Network, a branch of the U.S. Treasury Department. (You can find the form here.)

Please note: This rule applies to an entire group of travelers. For example, if the combined amount of money carried by everyone in your family totals more than $10,000, then you must declare it.

Failure to report could result in your money being confiscated, and you could be fined or charged with a crime.

There is no such requirement when you leave the U.S. with a large amount of cash but be sure to check your destination country’s rules. Every nation is different.

Advice for travelers

In general, travel experts caution against carrying excessive amounts of cash, given the abundance of credit cards, ATM machines and currency exchanges in airports. For those who do, they advise bringing a receipt, pay stub, bank book or other documentation that accounts for the source of the money.

There have been occasions when Transportation Security Administration officers have spotted thousands of dollars in carry-on luggage during the screening process at commercial airports and called in local law enforcement officials because they deemed the money suspicious.

Local police and federal agents—typically the FBI, Customs or the Drug Enforcement Agency—may then ask travelers where the money came from and why they’re carrying it, and possibly even seize it if they are unsatisfied with the answers they receive.

These interactions have prompted debate about whether TSA is allowed to flag large amounts of money, how law enforcement should react when asked to intervene and what rights travelers have in those situations.

For the traveler just trying to catch a flight, however, having proof that the money was acquired legally can help allay concerns and allow them to continue on their journey.