Reptiles (Reptilia) are vertebrates that have achieved a great variety of external morphology during their evolution process and have subsequently managed to fully adapt to terrestrial life. Nowadays they are more frequently encountered as pets. The morphological characteristics of the reproductive system and sexual dimorphism require a special attention, along with adequate conditions of upkeep and nutrition, in order to distinguish males from females for breeding purposes in addition to prevent aggressive behaviour between animals in captivity and unwanted breeding. The function of the reproductive system of male reptiles is the production of male reproductive cells and their transmission to the female reproductive system, and it consists of the testicles, excretory ducts and an intromittent organ. The female reproductive system consists of the ovaries and fallopian tubes, that are even numbered in most species. In some reptile species however, males and females can be distinguished on the basis of secondary sexual characteristics. Males and females usually vary in size, colour, length of the tail and claws, the colour of the iris, the curvature of the shell in turtles and developed femoral pores in lizards. Secondary sexual characteristics are often not present in a large number of reptiles, and in the majority of the offspring it is almost impossible to determine gender based on external morphology. Gender in this case is determined by other methods of determination, such as cloacal probing, digital palpation of cloaca, eversion of the hemipenis, ultrasonography, radiography and endoscopy.