Turtles (Testudines or Chelonia) are the oldest living reptiles on Earth, and today there are about 330 different species known, six of which are recorded in Croatia. Given the possibility of sagging the neck, we divide them into: hidden-neck (Cryptodira) turtles and sideneck (Pleurodira) turtles . Considering the living conditions of turtles, we divide them on terrestrial, freshwater and sea turtles. Terrestrial species are mostly herbivores, and freshwater and sea turtles are carnivorous and omnivorous. Terrestrial turtles have stocky legs that are adjusted for walking, freshwater have fingers connected with webbed feet, and sea turtles have adapted to life on the sea and developed limbs that resemble flippers.. The turtle armor represents a physical protection, but it also performs physiological functions. It is made of two parts: dorsal (carapace) and ventral (plastron). On the front and the back, there are openings for the insertion of the head, limbs and the tail in case of danger. Based on the texture, color, size of armor, and histology of long bone, we can estimate their age, origin and lifestyle. The methods used for this purpose are skeletochronology and the numbering of the growth zone of the scutes on plastron and carapace. With the development of turtles, the respiratory system has evolved and today it is anatomically and physiologically adapted to turtle life (water and land). The respiratory system is located within the armor, and the breathing role of the muscles is still in the early development of the body. Unlike mammals, the basic respiratory units are faveoles. The urinary and reproductive system in the turtle as well as in the other reptiles are separated. The products of the reproductive and urinary system are emptied in cloaca (common urogenital sinus). The male sexual system of the turtle serves for the production of male sex cells and consists of testicles, excretory canals and copulatory organs, and the female sexual system consists of the ovaries and the oviduct.