There are three large and very general groups of turtles. Sea turtles are found in large bodies of saltwater. Owing to Illinois' inland location, no members of this group are native to this state. All Illinois turtles belong to the group known as True Turtles (or just Turtles). Most species in this group have adaptations which make them well-suited to a primarily aquatic life in ponds, streams, and rivers. Only two Illinois turtle species, the Eastern Box Turtle and the Ornate Box Turtle, are more suited to a mainly terrestrial (land-based) life. These species are superficially like members of the last turtle group, the Tortoises, but all true tortoises belong to the family Testudinidae.

It is known that turtles have a good sense of smell and color vision. They are particularly able to distinguish yellows and reds. These well-developed senses serve them well in their search for food. Their hearing seems tuned to a range of low-frequency sounds, and they are able to detect very minute vibrations in the ground or in water. This is why turtles basking on a log in a pond will often dive away when anyone approaches the edge of the water.