Yes, you can bring an ice pack on a plane, but its approval by security agencies like the TSA depends entirely on its physical state at the time of screening. According to the "3-1-1" liquids rule, gels and liquids are restricted to 3.4-ounce containers; however, ice packs are an exception if they are completely frozen solid. If the ice pack has any visible liquid inside—meaning it has started to melt or contains "slush"—it will be treated as a liquid and must either be 3.4 ounces or less, or it will be confiscated. This rule exists because frozen solids provide a different X-ray signature than liquids, which could potentially hide prohibited substances. There is an additional "medical necessity" exception: if you are using the ice pack to keep life-sustaining medication (like insulin) or breast milk/baby food cool, it is generally allowed even if it is partially melted. In these cases, you must declare the item to the TSA officer at the start of the screening process for manual inspection. For general food cooling, the best strategy is to use a high-quality insulated bag and ensure the packs are rock-solid when you reach the front of the line. Once you are past security, you can often ask a restaurant or flight attendant for fresh ice to replenish your cooler if needed.