Alligators are characterized by a wider snout and eyes more dorsally located than their crocodile cousins. Both living species also tend to be darker in color, often nearly black but color is very dependent on the water. Algae-laden waters produce greener alligators; alligators from waters with a lot of tannic acid from overhanging trees are often darker (although the Chinese alligator has some light patterning.) Also, in alligators only the upper teeth can be seen with the jaws closed (in contrast to true crocodiles, in which upper and lower teeth can be seen), though many individuals bear jaw deformities which complicate this means of identification. The eyes of a large alligator will glow red and those of a smaller one will glow green when a light is shined on them. This fact can be used to find alligators in the dark.

An average American alligator's weight and length is 800 lbs. and 13 feet long. According to the Everglades National Park website, the largest alligator ever recorded in Florida was 17 feet 5 inches long (5.3 meters). The largest alligator ever recorded measured 19 feet 2 inches (5.8 meters) and was found on Marsh Island, Louisiana. Few of the giant specimens were weighed, but the larger ones could have exceeded a ton in weight. The Chinese Alligator is smaller, rarely exceeding 7 feet (2 meters) in length.

An alligator's lifespan is usually estimated in the range of 50 years or more. A specimen named Muja has resided in the Belgrade Zoo in Serbia since 1937, making it at least 70 years old. And in Riga Zoo, Latvia an alligator has been living since 1935.