Bicycle messengers (also known as bike or cycle couriers) are people who work for courier companies (also known as messenger companies) carrying and delivering items by bicycle. Bicycle messengers are most often found in the central business districts of metropolitan areas. Courier companies use bike messengers because bicycle travel is less subject to unexpected holdups in city traffic jams, and is not deterred by parking limitations, fees or fines in high-density development that can hinder or prevent delivery by motor vehicle, thereby offering a predictable delivery time.

Because bicycle thefts are prevalent in many cities, a lock to secure the bike during deliveries is essential. U-locks are also popular. Messengers typically carry basic tools, weather-proof clothing and a street map. It is also common for messengers to carry storage clipboards for manifests, receipts, and/or other logistic documents.

Almost immediately after the development of the pedal-driven velocipede in the 1860s, people began to use the bicycle for delivery purposes. Early bicycle messengers included couriers employed by the Paris stock exchange in the 1870s. During the bicycle boom of the 1890s in the United States, Western Union and American District Telegraph Company employed a number of bicycle telegraph boys in Omaha, Nebraska, New York City, San Francisco, and other large population centers.

One of the earliest recorded post-war American bicycle courier companies was founded by Carl Sparks, in San Francisco, in 1945. According to the San Francisco Bicycle Messenger Association, "Sparkie's went on to become Aero, which was bought out in 1998 [and] later absorbed into CitySprint." By the late 1970s, there were well-established companies offering bicycle messenger services in many major cities in the U.S.

In Europe, the bicycle had fallen out of favour as a means of delivery in the third quarter of the 20th century. It was not until 1983 that bicycle messengers made their reappearance in Europe. London's "On Yer Bike" and "Pedal-Pushers" were pioneers of pedal over petrol, and the rest of the city's courier companies followed suit. By the late 1980s, cycle couriers were a common sight in central London and a British manufacturer named a range of mountain-bikes for them, the Muddy Fox 'Courier'. Entrepreneurs in continental Europe, some inspired by seeing couriers in the U.S. or in London, began to offer bicycle courier services in the late 1980s, and by 1993 there were sufficiently large numbers of bicycle couriers in Northern Europe and North America that over 400 attended the inaugural Cycle Messenger Championships in Berlin, Germany. Bicycle messengers have not become common in southern Europe, the heartland of world competitive cycling. There are very few bicycle couriers in Portugal, France, Spain, or Italy. Outside Europe and North America, there are now large bicycle messenger services in Japan, and also in New Zealand and Australia. More recently, several companies started offering bike messenger services in Central and South America, specifically in México City, México; San José, Costa Rica; Bogotá, Colombia; Buenos Aires, Argentina; Santiago, Chile; Rio de Janeiro and São Paulo, Brazil.

Messengers carry a huge variety of items, from things that could not be sent by digital means (corporate gifts, original artwork, clothes for photoshoots, original signed documents) to mundane items that could easily be emailed, albeit without the air of importance attached to an express courier delivery. Messengers deliver digital content on optical media or hard disks because, despite high speed broadband connections, companies find it easier to send a disc than to work out how to transmit larger amounts of data than an email account can handle. Legal documents, various financial instruments and sensitive information are routinely sent by courier, reflecting a distrust of digital cryptography.

Commentators have claimed that technological innovation will significantly reduce the demand for same-day parcel delivery, predicting that the fax machine, and then the internet, would render the messenger business obsolete. There is still a demand for fast courier services, but there is certainly some truth in the predictions. Reliable data specifically relating to bicycle messenger occupational statistics is hard to find: the U.S. Department of Labor statistics does not track bicycle messengers specifically, and does not include "independent contractors" in statistics referenced for this industry occupation, but reports indicate the business is shrinking.

The gradual acceptance of electronic filing by U.S. courts has had a negative effect on the market. In San Francisco, bike messengers report a smaller work force coupled with decreased earnings. In New York City alone, the number of messengers dropped by approximately 1,000 between 1998 and 2008.