1. They are ectothermic vertebrates.

2. Their skin has scales, but no hair or feathers.

3. They have three-chambered hearts (except for alligators and crocodiles, which have four-chambered hearts).

4. They have claws on their toes (except those which do not have legs, such as legless lizards).

5. They are the first animals, in evolution, to develop the amniotic egg.  This allows reptiles to lay eggs on land.