Marine mammals are a diverse group, including the cetaceans (whales, dolphins and porpoises), pinnipeds (seals, sea lions and walrus), sirenians (manatees and dugong) and some carnivores (polar bear, marine and sea otter). These animals face a myriad of human threats, such as chemical and noise pollution, marine debris, ship strikes, prey depletion, habitat degradation, direct exploitation, and bycatch in fisheries. No marine mammal populations are entirely unaffected by human activities, and two species (Caribbean monk seal and Chinese river dolphin) have been declared extinct in recent decades. A third, the vaquita, as well as several others, teeter on the brink of extinction. We will discuss the varied threats to marine mammals as well as measures in place to protect them, including actions that we as citizens can take to contribute to these efforts.