People often use the words turtle and tortoise interchangeably, but these reptiles have distinct differences: Turtle shells are typically more flattened and not as deeply domed as tortoise shells, but there are always exceptions to these rules. In many turtle species, females are larger than males, while generally speaking, tortoise males are larger than females. Most turtles are aquatic or semi-aquatic, while tortoises live on land. Tortoises, however, are often good swimmers. Turtles are among the most endangered vertebrates. Habitat destruction, illegal collection, water contamination, and massive harvesting of wild turtles are decimating the world’s turtle population. Learn more about turtles and tortoises at the Smithsonian's National Zoo and sea turtles at the Smithsonian's Ocean Portal.