A mule or courier is someone who personally smuggles contraband across a border (as opposed to sending by mail, etc.) for a smuggling organization. The organizers employ mules to reduce the risk of getting caught themselves. Methods of smuggling include hiding the goods in vehicles or carried items, attaching them to one's body, or using the body as a container.

In the case of transporting illegal drugs, the term drug mule applies. Other slang terms include Kinder Surprise and Easter Egg. Small-scale operations, in which one courier carries one piece or a very small quantity, are sometimes called the ant trade.

Methods of smuggling include hiding the goods in a large vehicle, luggage, or clothes. In a vehicle, the contraband is hidden in secret compartments.

Sometimes the goods are hidden in the bag or vehicle of an innocent person, who does not know about the contraband, for the purpose of retrieving the goods elsewhere.

Some contraband is legal to possess but is subject to taxes or other import restrictions, such as second-hand clothes and computers, and the purpose of the smuggling is to get around these restrictions. In this case, smuggling may be done in plain sight, in smaller quantities, so that a suitcase full of used clothes or a new computer can be passed off as a personal possession rather than an importing business.

The practice of transporting goods outside or inside of the body is called body packing. This is done by a person usually called a mule or bait. The contraband is attached to the outside of the body using adhesive tape, glue, or straps, often in such places as between the cheeks of the buttocks or between rolls of fat. Other inconspicuous places, like the soles of cut out shoes, inside belts, or the rim of a hat, were used more often prior to the early 1990s. Due to increased airport security the "body packing" method is rarely used any more.

Some narcotics-trafficking organizations, such as the Mexican cartels, will purposely send one or two people with drugs on the outside of their body to be caught, so that the authorities are preoccupied while dozens of mules pass by undetected with drugs inside their body. However, even these diversionary tactics are becoming less prevalent as airport security increases.

This is often done using a mule's gastrointestinal tract or other body cavities as containers. Swallowing has been used for the transportation of heroin, cocaine, and MDMA (ecstasy). A swallower typically fills tiny balloons with small quantities of a drug. The balloons may be made with multilayered condoms, fingers of latex gloves, or more sophisticated hollow pellets. One smuggling method involves swallowing the balloons, which are recovered later from the excreted feces. Alternatively, the balloons may be hidden in other natural or artificial body cavities – such as rectum, colostomy, vagina, and mouth – although this method is far more vulnerable to body cavity searches. A drug mule may swallow dozens upon dozens of balloons. The swallower then attempts to cross international borders, excrete the balloons, and sell the drugs.