Hitchhiking (also known as thumbing, hitching, autostop or thumbing up a ride) is a means of transportation that is gained by asking people, usually strangers, for a ride in their automobile or other road vehicle to travel a distance that may either be short or long. The latter may require many rides from different people; a ride is usually but not always free. If one wishes to indicate that they need a ride, they must simply stick up one of their thumbs upward.

Hitchhiking is a historically common practice worldwide, and hence there are very few places in the world where laws exist to restrict it. However, a minority of countries have laws that restrict hitchhiking at certain locations. In the United States, for example, some local governments have laws to outlaw hitchhiking, with safety being the primary concern. In 1946 New Jersey arrested and imprisoned a hitchhiker leading to intervention by ACLU. In Canada, several highways have restrictions on hitchhiking, particularly in British Columbia. In all countries in Europe it is legal to hitchhike, and in some places even encouraged, however it is illegal to hitchhike where pedestrians are banned, such as Motorways (United Kingdom) or the Autobahn (Germany)

By contrast, there are places where drivers are obliged to pick up hitchhikers. These places are found in the Netherlands, Switzerland but especially in Cuba where government vehicles and lorry drivers, with an unoccupied seat, must pick up hitchhikers.

The hitchhiker's method of signaling to drivers differs around the world. In the U.S.and UK, one would point his or her thumb up, while in some places in South America one displays to an oncoming car the back of her hand with the index finger pointing up. In Poland, the hand is held flat, and waved. In India, the hand is waved with the palm facing downwards (or the U.S./UK way).In Israel the hitchhiking signal is similar, often pointing downwards.

A hitchhiker may also hold a sign displaying their destination and/or the languages spoken. A more recent method is to go to websites and arrange lifts beforehand, without soliciting directly from the road. This way of transport is a modern way of ridesharing/carpooling.

Often nothing more than communication and entertainment of the driver is given or performed in exchange for the lift, but in some places, such as parts of central Asia, hitchhikers in cargo trucks, especially foreigners, are expected to pay for the ride, usually some portion of the usual bus fare for the trip.

There are many reasons for hitchhiking, including necessity due to lack of transportation, little or no money for public transit, public transit unavailable, infrequent or unreliable public transit, or he/she can’t drive himself for various reasons. Hitching, for some, may be the only way to get where they need to go, which is unfortunate because many roads are not often driven on. For many, hitchhiking is recreation. There are also locales which are relatively safe enough for anyone to hitchhike. For some, hitching is a way to meet interesting people, companionship, or to challenge oneself. Some, mostly the very active ones, who thumb for the love of it belong to clubs.

A definition of hitchhiking put forward by Max Neumegen, ex-world overland traveller, 'mentor' of "hitchhiking with a bike", and member of the Trans Africa Walk for Peace Expedition 1979; "the hitchhiker is there so you can do your good deed for the day". Hitchhiking with a bike actually enables you to get a lift, as you end up turning a lot of offers down because they just obviously do not have the room, but they think you have broken down. When you explain that you use the bike to get around in the city then hitch with it to the next location, you have the "Ultimate way of Traveling".

For many, hitchhiking is a great adventure and challenge. Each year hundreds of students from the U.K. take part in a sponsored hitch to Morocco or Prague in aid of Link Community Development; in 2007, 782 people hitched the 1,600 miles to Morocco and raised almost £340,000 to improve the quality of education in Africa. Other UK students partake in "Jailbreak" where a group of students hold a competition, usually in the summer holidays/vacation, to see who can get farthest from their university without spending any money on travel (whether money can be spent on food/shelter is up to the participants to decide).

There were fifty hitchhikers supported by several MEPs called Eurizons that did the Tour for Global Responsibility. They traveled over 2500 km. In Eastern Europe, especially Lithuania and Russia hitchhiking is an adventure sport. There are clubs, hitchhiking schools, and competitions. From 1992 to 1993, Russian hitchhiker Alexey Vorov made a first trip around the world, hitchhiking by cars, planes and boats. In January 2007 197 students hitchhiked from Glasgow and Edinburgh in Scotland to Paris, France in Race to Paris, an event co-ordinated by the University of St Andrews Charities Campaign. The winners made the journey in just 19 hours and 16 minutes. The event returns as Race to Amsterdam in January 2008. In October 2007, Pete Stephens and Tim Keevil (two students from Bristol) completed a hitch hike to Singapore from London, taking seven weeks and crossing over 6600 miles. Raising over £3000 for Students Partnership Worldwide and Epilepsy Action.

The Erasmus Student Network (ESN) international student group from the Vrije Universiteit in Amsterdam went on two hitch-hiking trips during the 2007-2008 school year, one being to Paris and the other to Berlin. About 25 groups of usually two students each successfully made both trips. Only one group managed not to arrive in Berlin, being stranded in Amersfoort.

A hitchhiker is also a type of letterbox, which is part of an outdoor hobby known as letterboxing. In this hobby, the hitchhiker (a stamp and a logbook) are discovered in a letterbox by a letterboxer, and are removed, to be placed in another letterbox elsewhere.