Reptiles are one of the best animal groups to study interspecific and intraspecific interactions. Observing them gives us an insight into a very complex world where each individual struggles for existance, whether it is nutrition or survival. Individuals of different species often use the same resources, which will cause interspecific competition, resulting in interspecific aggression and competitive exclusion. The process of competitive exclusion occurs between the species Podarcis siculus and Podarcis melisellensis that came into contact at the Adriatic coast in Croatia. Furthermore, in the example of the species Podarcis melisellensis and Dalmatolacerta oxycephala, it is shown that interspecific aggression occurs if individual of one species enters the territory of the other. A large number of animals, including reptiles, uses body colaration to warn and repel possible attackers or predators. This is also the case of Urosaurus ornatus whose neck coloring indicates the aggressiveness and dominance of an individual. Also, dominance and aggression, as well as body size, are important in the territory occupation, and experimental evidence for this is shown on juvenile speciments of the species Psammodromus algirus. All these examples suggest that animal behavior is very unpredictable and every individual responds to danger in a different way. This is why the outcome of competitive responses is often impossible to predict.