Free-living copepods of the orders Calanoida, Cyclopoida, and Harpacticoida typically have a short, cylindrical body, with a rounded or beaked head, although considerable variation exists in this pattern.

The second pair of cephalic appendages in free-living copepods is usually the main time-averaged source of propulsion, beating like oars to pull the animal through the water.

Some copepods have extremely fast escape responses when a predator is sensed, and can jump with high speed over a few millimetres.

Many species have neurons surrounded by myelin (for increased conduction speed), which is very rare among invertebrates (other examples are some annelids and malacostracan crustaceans like palaemonid shrimp and penaeids).

Some copepod females solve the problem by emitting pheromones, which leave a trail in the water that the male can follow.

One foraging strategy involves chemical detection of sinking marine snow aggregates and taking advantage of nearby low-pressure gradients to swim quickly towards food sources.

Many benthic copepods eat organic detritus or the bacteria that grow in it, and their mouth parts are adapted for scraping and biting.

Herbivorous copepods, particularly those in rich, cold seas, store up energy from their food as oil droplets while they feed in the spring and summer on plankton blooms.

They are usually the dominant members of the zooplankton, and are major food organisms for small fish such as the dragonet, banded killifish, Alaska pollock, and other crustaceans such as krill in the ocean and in fresh water.

C. glacialis inhabits the edge of the Arctic icepack, especially in polynyas where light (and photosynthesis) is present, in which they alone comprise up to 80% of zooplankton biomass.

Many planktonic copepods feed near the surface at night, then sink (by changing oils into more dense fats)[21][22] into deeper water during the day to avoid visual predators.

[25] They attach themselves to bony fish, sharks, marine mammals, and many kinds of invertebrates such as corals, other crustaceans, molluscs, sponges, and tunicates.

[30] The copepod Calanus finmarchicus, which dominates the northeastern Atlantic coast, has been shown to be greatly infected by this parasite.

[29] Blastodinium-infected females of C. finmarchicus exhibited characteristic signs of starvation, including decreased respiration, fecundity, and fecal pellet production.

[28] Underdeveloped or disintegrated ovaries and decreased fecal pellet size are a direct result of starvation in female copepods.

Blastodinium parasitism is not lethal, but has negative impacts on copepod physiology, which in turn may alter marine biogeochemical cycles.

Freshwater copepods of the Cyclops genus are the intermediate host of the Guinea worm (Dracunculus medinensis), the nematode that causes dracunculiasis disease in humans.

[32] Despite their modern abundance, due to their small size and fragility, copepods are extremely rare in the fossil record.

The oldest known fossils of copepods are from the late Carboniferous (Pennsylvanian) of Oman, around 303 million years old, which were found in a clast of bitumen from a glacial diamictite.

Though most of the remains were undiagnostic, at least some likely belonged to the extant harpacticoid family Canthocamptidae, suggesting that copepods had already substantially diversified by this time.

[34][35] Transitions to parasitism have occurred within copepods independently at least 14 different times, with the oldest record of this being from damage to fossil echinoids done by cyclopoids from the Middle Jurassic of France, around 168 million years old.

[36] Live copepods are used in the saltwater aquarium hobby as a food source and are generally considered beneficial in most reef tanks.

[39] Copepods have been used successfully in Vietnam to control disease-bearing mosquitoes such as Aedes aegypti that transmit dengue fever and other human parasitic diseases.

Trials using copepods to control container-breeding mosquitoes are underway in several other countries, including Thailand and the southern United States.

The presence of copepods in the New York City water supply system has caused problems for some Jewish people who observe kashrut.

When a group of rabbis in Brooklyn, New York, discovered these copepods in the summer of 2004, they triggered such debate in rabbinic circles that some observant Jews felt compelled to buy and install filters for their water.

The Nickelodeon television series SpongeBob SquarePants features a copepod named Sheldon J. Plankton as a recurring character.