Calanoids have been the most successful of all copepods in colonizing all parts of the pelagic environment in both marine and freshwaters. Different ocean depths are characterised by different species from the surface-living plankton to the depths of the bathyal zone. There are estuarine, coastal, and oceanic species. Some species may be restricted to particular water masses, or oceans, or may be widespread in tropical and subtropical waters. A few families live in association with the seafloor in both shallow, continental shelf, and deep ocean waters, well as in anchialine caves (e.g. Huys and Boxshall, 1991; Fosshagen et al., 2001).

The calanoid copepod body is divided into two major parts: the prosome and the narrower urosome with the major articulation occurring between the fifth pedigerous and the genital somite (gymnoplean position) (Huys and Boxshall, 1991; see figure below ). The prosome encompasses the cephalosome and 5 pedigerous somites. The urosome is formed from 4 free somites in female and 5 free somites in male (although there may be fusions between these somites) and is terminated by a pair of caudal rami. The female genital double-somite is composed of the fused genital somite and the first abdominal somite. The rostrum is usually fused to cephalosome. The pairs limbs of the cephalosome are, from anterior to posterior, antenna 1 (antennule), antenna 2 (antenna), mandible, maxilla 1 (maxillule), maxilla 2 (maxilla), and maxillipeds. When detail of the morphology of the limbs is referred to in the key, the part concerned is indicated by a red box. Swimming legs 1-4 are biramous with a basic 3-segmented plan for each ramus (although some of these segments may be fused) and branches of each pair of legs are joined by an intercoxal sclerite. The fifth legs are sexually dimorphic. Sometimes the female fifth legs are absent, but when present, can be biramous or uniramous; they are always present in male, and can be of a simple uniramous form or large, complex and highly asymmetrical.

Although calanoids have a naupliar eye, their mode of sensory perception is generally non-visual. The cephalosome appendages, especially antenna 1, are equipped with mechano- and chemosensory setae that allow the copepod to distinguish mates from prey and predators (e.g. Boxshall et al., 1997). The Calanoida are primarily suspension feeders eating mainly phytoplankton and protozoans. They use their mouthparts to create water currents that bring food particles towards the copepod. Smaller particles are then captured passively and are directed towards the mouth by setae on the maxillipeds, maxillae 1 and 2, while larger particles are individually grasped by 'fling and clap' movements of maxilla 2 that grasp both the particle and a packet of water surrounding it and remove the water by an inward squeeze (e.g. Koehl and Strickler, 1981; Price et al., 1983). Many families contain genera that are carnivorous with many modifications of the mouthparts and behaviour to make this mode of feeding successful. The mandible has a strongly sclerotized gnathobase with narrow, saber-shaped teeth on the cutting edge. Usually the ventral teeth are sharper in raptorial species. The fewer the number of teeth the greater the degree of carnivory (e.g. Ohtsuka et al., 1996). Maxilla 1, maxilla 2 and the maxilliped are all modified and usually have claw-like setae. The mandible of heterorhabids appears to be able to inject an anaesthetic through the tubular lumen of the ventralmost tooth into the prey (Nishida and Ohtsuka, 1996). Carnivorous or detritivorous forms occupy deeper water layers down to the deepest trenches.

Reproduction is sexual and sperm are transferred from male to female in a sac–like spermatophore (e.g. Mauchline, 1998). Egg sacs are probably not an ancestral condition of Copepoda. Most Calanoida lack egg sacs as eggs are usually laid freely into the water. Nevertheless, in some families the eggs are carried in one or two egg masses, sacs, or strings until hatching. Under favourable conditions females of some species may produce tens to hundreds of eggs in a life time. The egg hatches into a nauplius larva and the life cycle typically includes 6 naupliar stages and 6 copepodite stages, the last of which is the adult stage. There is a marked metamorphosis between the last nauplius and the first copepodite stage. Development times from egg to adult are typically in the order of 1 to 6 weeks, but may take several months, and the lifespan of adults may be from one to several months. Developmental times are markedly affected by temperature and food levels. Some Calanoids have resting stages that enable avoidance of detrimental environmental conditions and dispersal. Calanoids resting eggs have a thick shell and which can survive extended periods of dormancy.

The Calanoida comprise 43 families with about 2000 species (Boltovskoy et al., 1999). Recently the study of marine caves has revealed calanoid copepods of great importance to the study of evolutionary relationships between the various groups of copepods, as some of them appear to be amongst the most primitive forms (e.g. Huys and Boxshall, 1991).

In both marine and freshwaters worldwide, abundant Calanoida are a vital link in the food web that leads from minute algal cells and small protozoans (e.g., Bradford–Grieve et al., 1999) to the largest fishes and some whales. Many commercial and non–commercial marine fish (and some crustaceans) are utterly dependent on copepods as a food source during a portion of their larval life. For example, in New Zealand it has been shown that the larvae of hoki (Macruronus novaezelandiae), which form the basis of the largest New Zealand fishery, feed on copepod adults (e.g. Calocalanus) and copepodites almost exclusively (Murdoch, 1990). Copepods can be so abundant that their faecal pellets, produced at a rate of several per hour, are an important source of food for detritus feeders.

Further information may be obtained from publications such as Mauchline (1998) and Huys and Boxshall (1991) and the references therein.