It is not "bad" or illegal to fly with large sums of cash, but it does come with significant risks and administrative requirements that you must navigate carefully. Within the United States, there is no legal limit on the amount of cash you can carry on a domestic flight, and you are not required to declare it to TSA. However, TSA has the authority to notify law enforcement if they discover a large amount of cash that they suspect may be tied to criminal activity. This can lead to "civil asset forfeiture," where the money is seized even if you aren't charged with a crime. On international flights, the rules are much stricter: you MUST declare any amount exceeding $10,000 (or the equivalent in foreign currency) to Customs and Border Protection. Failure to declare this can lead to the total seizure of the funds, heavy fines, or even criminal charges. Aside from the legalities, carrying a lot of cash makes you a target for theft and offers no protection if the money is lost, unlike a credit card or traveler's checks. If you must travel with a large sum, keep it in your carry-on (never checked luggage) and have documentation ready to prove its legitimate source.