When crossing international borders, you must declare any items that are restricted, taxable, or potentially hazardous to the destination's environment. This primarily includes food and agricultural products like fresh fruits, vegetables, seeds, and most meat or dairy items, which could harbor invasive pests or diseases. You must also declare currency exceeding $10,000 USD (or the local equivalent) to comply with anti-money laundering laws. Other mandatory declarations include "controlled substances" (even if they are prescription medications, you should have the doctor's note ready), commercial goods intended for resale, and high-value purchases made abroad that exceed your "personal exemption" limit (often $800 for U.S. residents). Failure to declare even a single piece of fruit can lead to immediate fines ranging from $300 to $1,000 and the revocation of trusted traveler statuses like Global Entry. When in doubt, it is always better to declare an item to the customs officer; they will simply tell you if it is allowed, whereas hiding it can lead to legal consequences and seizure.