The oceans are our planet's most distinctive and imposing natural habitat. They cover seventy-one percent of its surface; support a remarkably diverse and exquisitely adapted array of life forms; and possess many of Earth's most significant, intriguing, and inaccessible ecosystems. In an era in which humans are significantly altering the global environment, the oceans are undergoing rapid and profound changes. The study of marine biology is thus taking on added importance and urgency as people struggle to understand and manage these changes to protect our marine ecosystems. Healthy oceans produce half of the oxygen we breathe; stabilize our climate; create ecosystems that protect our coasts from storms; provide us with abundant food; and host diverse organisms that provide us with natural products for medicine and biotechnology. In this Very Short Introduction, marine biologist Philip Mladenov provides an accessible and up-to-date overview of marine biology, offering a tour of marine life and marine processes that ranges from the unimaginably abundant microscopic organisms that drive the oceans' food web to the apex predators that we exploit for food; from polar ocean ecosystems to tropical coral reefs; and from the luxurious kelp beds of the coastal ocean to deep-ocean hydrothermal vents where life exists without the energy of the sun.