Turtles courtship and mating commonly occur in the spring and fall. The courtship of sliders (Trachemys scripta), painted turtles (Chrysemys picta), false map (Graptemys pseudogeographica), and common map turtles (Graptemys geographica) consists of the relatively small male swimming backward, ahead of the female, while fanning his elongated front claws in front of her snout. In those species where the female is the smaller sex, the male typically uses aggressive behavior to immobilize the female so that he can obtain a mating position at the back edge of her shell. He may, for example, bite at the females head and legs. In box turtles, the females shell is quite high, requiring the male to hook the claws of his hind feet beneath her shell and then rear up into a vertical stance in order to mate.

All turtles must nest on land. Egg-laying typically occurs between mid-May and early July. A nest is usually a flask-shaped hole scooped out with the females back feet. After egg-laying, the female again uses her back feet to pull dirt into the hole and pack it down. When the nest is covered, she abandons it, never returning to see her young. The yellow mud turtle (Kinosternon flavescens) female is an exception. She digs a nest burrow with her front legs and then remains with the eggs in the burrow for several days to two weeks. Nevertheless, she is long gone by the time the eggs hatch. Most Illinois turtles lay oval eggs, but softshells and snapping turtles lay spherical eggs. Small species, such as the spotted turtle (Clemmys guttata), may lay only three to five eggs in a nest, while the larger snapping turtle (Chelydra serpentina) lays 20 to 40 eggs. Spotted, snapping, and Blandings (Emydoidea blandingii) turtles lay eggs once per year. Others, including the common musk turtle (Sternotherus odoratus), map turtles, the painted turtle, and the slider, commonly nest two or three times per year, at two to three week intervals.

The life cycle of the eastern box turtle (Terrapene carolina) is typical of most turtles. After mating, the female digs a nest in the soil with her rear legs. She deposits the eggs, then uses her rear legs to cover them with dirt. Hatchlings must dig ther way out of the nest. The turtles reach adult state in five to seven years.