Cetaceans are one of the most unknown groups in the animal kingdom. Their study is very difficult due to their scarcity, their flighty behaviour, and their enormous displacements. In addition, some species feed in very deep waters and only emerge to breathe, or they live in hard to reach areas. However, few animals create so much wonder. What child or adult has not felt emotions when contemplating the spectacles of the killer whales, the pirouettes of the dolphins, or the immensity of the whales in the middle of the sea? Some of the authors of this book have been so moved to be surrounded by a group of fin whales that could not contain their tears. It is a difficult sensation to explain. One could try to summarise it by saying that these immense animals exhale peace. Seeing dolphins making capers around a boat or a group of fin whales suddenly emerging a few meters away is one of the greatest shows we can see in nature. In the presence of these animals, there is only a feeling of recollection. These are the moments when we see how small we humans are in this wonderful biosphere. Therefore, this book is a small tribute to these beautiful living beings. Something we owe them. We have to protect these animals, but we cannot do so without knowing them. In this text, the authors provide key information to understand the evolution of cetaceans, their patterns of movement and behavior, always under a conservation perspective. This book has been written by 15 leading specialists from three different countries and with different backgrounds ranging from biology to veterinary science. They are united by a common interest, a passion. The book does not focus on a single species, but covers studies with dolphins, sperm whales, and fin whales, as well as various methodological approaches that provide key data to understand the evolution of up to 50 different species. Many diverse aspects have been covered and, above all, new methodological approaches have been provided in areas such as acoustics, the study of spatial and feeding behavior, evolution, social interactions, and even blue economics. As if this were not enough, the authors wish to highlight the immense amount of data that has been used. The first chapter on fin whales is based on hundreds of sightings over several years, and dozens of satellite images on various environmental parameters. Chapter two covers hundreds of recordings of the sounds of humpback whales around the world. Chapter three has followed the sperm whale populations of the Strait of Gibraltar over several years with hundreds of sightings. Chapters four and five study the evolution of more than fifty species through hundreds of recordings using qualitative and quantitative approaches, respectively. The sixth chapter compares the behavior of two species of dolphins. Finally, chapter seven designs a new line of study for these animals: the blue economy.