The first stage of the alien's lifecycle consists of an egg. The eggs are complex organisms in themselves. They are responsible for maintaining life support for the larva for an indefinite amount of time, and must recognize a potential host and distinguish it from valid members of the nest. Eggs are initially created inside a queen alien. The egg itself is a leathery object 2.5 feet tall.

The next stage, hibernating inside one of these eggs, is a parasite commonly refereed to as a face-hugger. Concealed by the egg for protection, the face-hugger waits until the egg senses it is time to continue the life cycle. When a viable host is brought near a closed it triggers the contents of the egg to come to life. The egg opens from the top to reveal the creature within it. At this point the cycle transfers from the egg to the next phase as the face-hugger awakens. After sensing the nearby host, the face-hugger launches out at the organism and attaches itself by wrapping a long tail around its victim's neck and using long spider-like legs (like a spider, the face-hugger has 8 legs) to firmly grip the organism's head. The legs become almost fused with the head of the soon-to-be host, making it extremely difficult to remove. 

Developing inside the host, this stage of the lifecycle is refereed to as the embryo or spore stage. This begin before the facehugger has actually departed from the host. Immediately after the embryo has reached the body cavity suitable for gestation (from the transplanting of the facehugger's long tube), it begins to feed off of the host. Using the host's fluid and resources, the embryo grows and develops. This can take a period of anywhere from a few hours (as with Kane in Alien) or a few days (as with Ripley in Alien3). Even though the chest bursters that came out of them were both the same size, why the time difference? Taking into account the parallels between the aliens and an insect colony, two Alien hives will be in competition if they are close to each other. Therefore the incubation period of queens is higher to enable the unsuspecting host to move further from the original hive. There are beliefs that during this period, the embryo consults the host's DNA as to the best way of developing. According to this belief, the embryo would take on traits of the host and therefore become most suitable developed for the environment that it will live in after it has left the host.

When the embryo has completed its period of development inside of the host, it emerges into the next stage in its life. The burster has built up enough strength into order to tear out of the body cavity inside of the host. The host experiences a great deal of pain during this procedure, resulting in convulsions and loss of all rational thought. This process usually results in the death of the host, which in all practical means isn't all the greatest of way to ensure the reproduction of the species. It would be more efficient if the host were reusable for many aliens, but due to the alien’s violent nature, it is only fitting for it to cause death upon birth. Once free of the host, the chestburster is independent to live on it's own. About two feet long from head to tail, it emerges can emerge at different stages of development depending on host conditions, environment, and strength. Unlike the burster seen in Alien, the one in Aliens had arms, while the latter was merely a head and tail. Once again the chestburster seen in Alien3 was also different; it was more mature looking immediately  after its birth. Specifically, unlike the chestbursters of the previous movies, this one had legs, and a noticeable already developed inner jaw or tongue. The burster is very unprotected and vulnerable at this point.

This is the final stage in the development of the standard xenomorph. During it's growth stage it sheds at least one layer of skin, similar to a snake, in Alien, Brett finds a small piece of skin-like material about the size of the original burster on the ground. In the usual case this increase in mass would take much longer. In Alien, it would've needed to feast on all the crew and metabolise their body mass before it even grew to be half the size of a man. We can also assume that is sheds skin multiple times in its rapid growth. In Alien3, while cleaning out a vent shaft, one of the prisoners discovers something similar to Brett's finding shortly before getting his skull punctured. In this same scene, the alien doesn't appear to be fully mature, but definitely larger then the original burster proving that it doesn't instantaneously grow to full size. Whether the alien needs any food at all for growth is has not been fully determined. In Alien it obviously has no food source between birth out of Kane, and the death of Brett, where it has grown to full size.

Little is known about the Queen Alien. From Alien3 we know that a queen alien can be born in the same way as a regular alien, Some things that we do know about the queen: she has a much larger cranium than the usual alien and is slightly taller. The queen has the ability to create and lay eggs and she has the ability to survive without the extended abdomen.