Marine mammals are animals which have always fascinated us because of their size and the shapes they have adopted to survive. To watch cetaceans in their natural environment is one of the most wonderful spectacles that exist; they remind us that the sea contains a huge amount of resources and shelters numerous species. For their part, sirenians are a little-known group, but not less interesting than the previous one; besides, they are endangered species. In this book we will learn about the places where they live, the food they consume and about their enemies. We will also learn about the dangers they face, mainly because of the contamination and the irrational exploitation by humans of rivers and seas.