The marine environment is the largest, most important, and yet most mysterious habitat on our planet. It contains more than 99% of the world's living space, produces half of its oxygen, plays a critical role in regulating its climate, and supports a remarkably diverse and exquisitely adapted array of life forms, from microscopic viruses, bacteria, and plankton to the largest existing animals. In this unique Very Short Introduction, biologist Philip Mladenov provides a comprehensive overview of marine biology, offering a tour of marine life and marine processes that ranges from the polar oceans to tropical coral reefs, and from shoreline mollusks to deep-sea hydrothermal vents. Mladenov also looks at a number of factors that pose a significant threat to the marine environment and to many of its life forms-threats such as overfishing, coastal development, plastic pollution, oil spills, nutrient pollution, the spread of exotic species, and the emission of climate changing greenhouse gases. Throughout the book he successfully weaves around the principles of marine biology a discussion of the human impacts on the oceans and the threats these pose to our welfare.