Mammals, fascinating and diverse vertebrates, whose origin is unclear and is confused with the times in which dinosaurs and other reptiles lived. The objective of this book is to determine when they appeared, as well as to know the reptiles that gave them origin and the primitive groups of mammals that existed. Likewise, the climatic and geological changes that have influenced the appearance and disappearance of the different groups of these tetrapods are analyzed, up to the current monotremes, marsupials and placenta.