We are going to study how dinosaurs lived, how they diversified and dominated the planet for more than 150 million years. We will also become acquainted with other groups of reptiles that we call the dinosaurs’ cousins. These were also very successful; they lived in the same epoch and ruled the air and the water. We are talking about the Pterosaurs, Ichthyosaurs, Mosasaurs, Plesiosaurs and Pliosaurs. Finally, we will study the possible causes for the extinction of these animals which made possible that other group called mammals could develop and reach its present peak. Human beings are now one of the domineering species, but if they do not take care of the environment, they will vanish from earth as dinosaurs did.