An amphibious vehicle (or simply amphibian), is a vehicle capable of driving on land or flying as well as on (or under) a waterway. Amphibious vehicles include amphibious bicycles, ATVs, cars, buses, trucks, military vehicles, boats, aircraft, and hovercraft.
Classic landing craft are not amphibious vehicles as they do not offer any real land transportation at all, although they are part of amphibious warfare. Ground effect vehicles, such as ekranoplans, will likely crash on any but the flattest of landmasses so are also not considered to be amphibious vehicles.
Apart from the distinction in sizes mentioned above, two main categories of amphibious vehicles are immediately apparent: those that travel on an air-cushion (Hovercraft) and those that do not. Amongst the latter, many designs were prompted by the desire to expand the off-road capabilities of land-vehicles to an "all-terrain" ability, in some cases not only focused on creating a transport that will work on land and water, but also on intermediates like ice, snow, mud, marsh, swamp etc. This explains why many designs use tracks in addition to or instead of wheels, and in some cases even resort to articulated body configurations or other unconventional designs such as screw-propelled vehicles which use auger-like barrels which propel a vehicle through muddy terrain with a twisting motion.
Most land vehicles – even lightly armoured ones – can be made amphibious simply by providing them with a waterproof hull and perhaps a propeller. This is possible as a vehicle's displacement is usually greater than its weight, and thus it will float. Heavily armoured vehicles however sometimes have a density greater than water (their weight in kilograms exceeds their volume in litres) and will need additional buoyancy measures. These can take the form of inflatable floatation devices, much like the sides of a rubber dinghy, or a waterproof fabric skirt raised from the top perimeter of the vehicle, to increase its displacement.
Some of the earliest known amphibious vehicles were amphibious carriages, the invention of which is credited to the Neapolitan polymath Prince Raimondo di Sangro of Sansevero  in July 1770 or earlier, or Samuel Bentham whose design of 1781 was built in June 1787.
The first known self-propelled amphibious vehicle, a steam-powered wheeled dredging barge, named the Orukter Amphibolos, was conceived and built by United States inventor Oliver Evans in 1805, although it is disputed to have successfully travelled over land or water under its own steam.
Inventor Gail Borden, better known for condensed milk, designed and tested a sail-powered wagon in 1849. On testing, it reportedly tipped over 50 feet (15 m) from shore, from an apparent lack of ballast to counteract the force of the wind in the sail.
Some military vehicles are capable of "wading" using waterproof screens to keep the upper hull dry. In World War II the tanks following the Sherman DDs were given waterproofed hulls and trunking was fixed to the engine intakes and exhausts to allow them to come ashore from landing craft in shallow water. The Germans gave their Tiger tank a long snorkel, essentially a long tube on the commander's hatch that allowed it to wade through four metres of water.
The Leopard 2 tank can use a series of rings to create a long tube. This tube is then fitted to the crew commander's hatch and provides air and an escape route for the crew. The height of the tube is limited to around three meters.
Some civilian deep wading vehicles achieve their capability by means of legs or stilts to raise the body of the vehicle from its wheels. One example is the Sea tractor, a motor vehicle that can travel through shallow water, with driver and passengers on a raised platform.
The unique capability that distinguishes multi-unit vehicles from single unit ones, is the ability to help each other. According to a 1999 article in Military Parade magazine, multi-unit, all-terrain transport vehicles were first proposed by the British in 1913, and by the 1950s, over 40 types of articulated tracked vehicles (ATVs) were in production. The articulated tracked concept is chosen primarily for its combination of high maneuverability, cross-country abilities, and remarkable load-carrying capacity.
An amphibious aircraft or amphibian is an aircraft that can take off and land on both land and water. Fixed-wing amphibious aircraft are seaplanes (flying boats and floatplanes) that are equipped with retractable wheels, at the expense of extra weight and complexity, plus diminished range and fuel economy compared to planes designed for land or water only. Some amphibians are fitted with reinforced keels which act as skis, allowing them to land on snow or ice with their wheels up.
Floatplanes often have floats that are interchangeable with wheeled landing gear (thereby producing a conventional land-based aircraft) however in cases where this is not practical amphibious floatplanes, such as the amphibious version of the DHC Otter, incorporate retractable wheels within their floats.
Many amphibian aircraft are of the flying boat type. These aircraft, and those designed as floatplanes with a single main float under the fuselage centerline (such as the Loening OL and Grumman J2F), require outrigger floats to provide lateral stability so as to avoid dipping a wingtip, which can destroy an aircraft if it happens at speed, or can cause the wingtip to fill with water and sink if stationary. While these impose weight and drag, amphibious aircraft also face the possibility of these getting hit when operating from a runway. A common solution is to make them retractable as those found on the Consolidated Catalina however these are even heavier than fixed floats.
Amphibious aircraft are heavier and slower, more complex and more expensive to purchase and operate than comparable landplanes but are also more versatile. Even if they cannot hover or land vertically, for some jobs they compete favorably with helicopters and do so at a significantly lower cost. Amphibious aircraft can be much faster and have longer range than comparable helicopters, and can achieve nearly the range of land based aircraft, as an airplane's wing is more efficient than a helicopter's lifting rotor. This makes an amphibious aircraft, such as the Grumman Albatross and the Shin Meiwa US-2, useful for long-range air-sea rescue tasks. In addition, amphibious aircraft are particularly useful as "Bushplanes" engaging in light transport in remote areas, where they are required to operate not only from airstrips, but also from lakes and rivers.
An occasional problem with amphibians is with ensuring the wheels are in the correct position for landing. In normal operation, the pilot uses a checklist, verifying each item. Since amphibians can land with them up or down though, the pilot must take extra care to ensure they are correct for the chosen landing place. Landing wheels up on land may damage the keel (unless done on wet grass, a technique occasionally used by pilots of pure flying boats), while landing wheels down on water will almost always flip the aircraft upside down, causing substantial damage.