The most common roundworm parasites have similar direct life cycles. Females lay thousands of eggs, which pass out in the faeces of infected animals. If the environmental conditions of warmth and moisture are favourable, eggs deposited in the faeces will develop to the first larval stage (L¹) and hatch in several hours. Alternately, as with Toxocara and Toxascaris, larvae may remain in the egg and develop until eaten and released. If deposited on dry ground or if temperatures are low, the eggs develop more slowly or will not survive. Hatched larvae can thrive on the ground, feeding mainly on bacteria.

Larval growth is limited by a rigid skin, or cuticle. Larvae increase their size through the process of moulting. When a first-stage larva grows to the limits of its cuticle, it develops a second, larger cuticle underneath the first, then casts off the old one to become a second-stage larva (L²). The L² grows to its limits and moults again to become a third-stage larva (L³). The third-stage larvae will have the ability to infect cats. Once inside the animal, infective larvae become established in the site of predilection appropriate to their species and develop into adult worms. Life cycles of nematodes infecting cats often do not conform to the simple direct life cycles characteristic of many roundworm parasites of other species. Major differences occur in the development of infective larval stages and the patterns of migration these forms follow in the host.

Toxocara and Toxascaris nematodes are very common in cats throughout the world. Infective larval forms develop outside the host of eggs. Toxocara and Toxascaris second-stage larvae inside eggs are infective. Cats may be infected by ingesting these eggs. Digestion stimulates the hatching of larvae. Infective larvae may also be acquired by cats after first being ingested by transport or paratenic hosts, such as rodents. This occurs in the life cycle of Toxascaris and Toxocara. Paratenic hosts are specific for the nematode species. Ingestion and digestion of these organisms release infective larvae and the normal course of infection resumes.

Once inside the principal host, larvae may migrate through tissues before establishing themselves at the predilection site. Blood that is returned from body tissues to the heart is pumped to the lungs, so larvae that enter the circulation often pass through the lungs in a pattern known as tracheal migration. Infective larvae of Ancylostoma Uncinaria (not significant in New Zealand) may infect cats by penetrating their skin and entering blood vessels. Circulation then carries them to the lungs. Here they often migrate out into the air spaces and travel up the airways to the trachea, where they are coughed up and swallowed. Development to adult stages is completed in the small intestine. Infective larvae of Toxocara, after being swallowed, penetrate the gut wall and enter blood vessels. They then follow a tracheal migration.

Indirect life cycles occur with some nematodes in cats. Aelurostrongylus abstrusus is an example. The eggs form small nodules in lung tissue where they hatch. The larvae ascend the air passages, are swallowed and passed out in the faeces. These first stage larvae survive only about 2 weeks in the free state. For further development to occur they require snails or slugs.

Cats are infected when they eat these intermediate hosts.

Adult tapeworms in the small intestine of the principal host grow by generating proglottids from the scolex. Proglottids mature and after fertilisation, become gravid (enlarged and filled with eggs). The enlargement of segments as they mature results in the characteristic widening of the tapeworm body toward its end. Gravid proglottids break off from the end of the tapeworm and pass in faeces.

Eggs are released as the proglottids decompose, either within the animal or on the ground in the faeces. With some species of tapeworms, eggs will not appear in the faeces even when a heavy infection exists. Rather, intact proglottids are found. Neither the proglottids nor the eggs can infect cats. To become infective, eggs first must develop to the infective stage in intermediate hosts, such as arthropods or mammals. Because of the need for intermediate hosts, the life cycle of tapeworms is indirect.

The embryo that develops within the tapeworm egg is known as a hexacanth. When ingested by the intermediate host, it hatches and develops into an immature stage called a metacestode. Metacestodes of different tapeworm species show differences in structure. A metacestode is usually a fluid-filled cyst with one or more immature scolices (heads) growing on the inner lining. Infection of the principal host occurs when an intermediate host or part of the host tissue containing the immature metacestode is ingested. Digestion releases the scolices, which mature in cats and become adult tapeworms.