A courier service is an organization which offers special deliveries of packages, money, documents or information. Courier services usually boast faster delivery times than any alternative method of transporting documents, and many services in the modern world rely on them. The idea of couriers and a courier service has been around almost as long as civilization, with rulers in antiquity using couriers as a means to make new laws and edicts known throughout their lands. In the modern age of international business, the courier service has become a keystone of enterprise, even as emerging technologies such as the fax machine and Internet have rendered them less useful in some areas.

The largest courier service in the world is the United Parcel Service (UPS), which delivers more than 12 million packages globally each day. UPS has its roots as a courier service in the beginning of the 20th century, when it was known as the American Messenger Company. UPS survived the depression and the World Wars, and went on to thrive in the global age. Throughout the years it acquired other courier services, including the Motorcycle Messengers and a number of smaller European companies.

Federal Express (FedEx) and DHL are other well-known global examples of the courier service, both with their roots in the early 1970s. Though not nearly as large as UPS, each company controls sizable portions of the market. FedEx is particularly well-respected for its overnight courier service, the first of its kind. DHL originally focused exclusively on the route between Honolulu and San Francisco in the United States, but grew to encompass the Pacific Rim and spread to Europe and the Middle East.

Smaller courier services abound, particularly in cities. This type of courier service finds its niche in smaller packages and documents which need to be transported from one location in a city to another as quickly as possible. Couriers in these companies often ride bicycles or motorcycles in order to achieve the shortest turnaround times from pickup to delivery. Indeed, the bicycle messenger, whizzing through crowded traffic at unbelievable speeds with a satchel over one shoulder, has become a symbol of the youthful daredevilry of city life. In recent years, with the emergence of more sophisticated communications technologies and a harsh price competitiveness, the number of bicycle couriers in most cities has decreased significantly, but the market remains strong for those who continue to work.

There is also a type of courier service which utilizes the extra baggage allotment of airline passengers to transport documents quickly and easily, with less hassle, through the Customs office of a country. These courier services offer greatly discounted fares on commercial tickets to foreign locations in exchange for the use of the passenger's baggage space. The passenger is then given all the requisite paperwork, which they simply deliver to the Customs agent at the receiving end.

Though technology has replaced many traditional uses of the courier service, there still exists a constant need to transport physical goods and documents over both short and long distances as quickly as possible. Barring the development of some sort of far-fetched teleportation technology, it seems certain that the demand for a courier service will remain strong.