Jellyfish are found in all of the Earth's oceans, including the very deep sea, as well as shallow saltwater lakes. Various jellyfish species have a wide range of living preferences, with some preferring arctic waters, and others preferring warmer tropical waters. There are animals called hydrozoans that are closely related to jellyfish and live in freshwater lakes.

Most jellyfish exist well above the ocean floor, floating with the plankton, although there are species that spend their lives at the bottom of the ocean. Some are not able to swim and use their tentacles to "walk" across the ocean floor. Others migrate back and forth to the surface during the day, and then retreat back to the depths at night. If a jellyfish is removed from the water, it collapses and perishes, as the water provides its body with necessary structural support.

Some jellyfish species are kept in aquariums. Although the colored lighting and contrasted backgrounds make the jellyfish appear to glow, many of them are actually so transparent that they would be difficult to see in the wild. Because jellyfish depend on water currents to move them, professional jellyfish exhibits have to use precise pumps to keep the water moving, preventing jellyfish from becoming trapped in corners.