This seminar is about understanding and describing morphological and molecular data gathered from numerous fossil records in order to conclude about the origin of species of the modern Order Cetacea. In this sense, it is important to understand the biotic and the abiotic factors that had influenced distribution of species of this Order through its history. Morphological and molecular data supports theory that artiodactyls are the closest relatives of cetaceans. The earliest whales were Archaeocetes, a paraphyletic group that had appeared approximately 50 million years ago, and are best known from India and Pakistan. These basal whales were morphologically very diverse; many of them had well developed hind limbs and feet. Molecular data and fossil record are in agreement that baleen and toothed whales diverged from a common ancestor approximately 35 million years ago. The caudal fin oscillation mode (swimming mode of modern whales) evolved from an initial quadrupedal locomotors stage. This was followed by a pelvic phase (Ambulocetus), a caudal undulation phase (Kutchicetus), and the final adaptation of caudal oscillation, the way of swimming seen in Dorudontinae and all modern cetaceans. The earliest cetaceans, Pakicetidae, used the same sound transmission mechanisms as land mammals, their hearing sensitivity in water was poor. The sound reception system had pass through a number of anatomical specializations of the ear and lower jaw which has fat pad that acts as additional sound channel for conducting high – frequency sound to middle ear. Further transformations of ear and jaw of Odontoceti probably leaded to development of echolocation.