The Egg is roughly one meter in height and two feet wide with four pedal-like lips at the top of it. Eggs are laid by an Alien Queen and they have root-like tendrils that extend from their base. Newly-laid Eggs do not contain tendrils which suggest that they slowly develop over a period of time. Within an Egg is a Facehugger along with fluid and other materials to keep it alive. Once an Egg has fully developed, its tendrils draw in nutrients from the surrounding environment to sustain the parasite after the limited supply of material inside the Egg runs out. One problem that is unsolved is how the Facehugger's waste is disposed of since there must ultimately be a by-product, but the Egg may simply excrete it or turn it into more useable nutrients. An Egg's purpose is to detect a potential host, awaken the Facehugger inside it, and allow the Facehugger to leave upon locating the host. An Egg's life span is unknown, but people have speculated that it is able to stay alive for centuries since the Eggs inside the Derelict survived for at least 57 years.

The Facehugger is roughly one and a half meters in length (including its tail) with a long retractable tube-like organ that is thought to be its mouth. When a potential host is able to move around freely the Facehugger will use its tail to launch itself out of the Egg. If the Facehugger fails to attach itself to the host it will use its six legs or fingers to move around in which it can move surprisingly fast. It has been speculated that a Facehugger can climb on walls like its full-grown kin, but instead of climbing the Facehugger may use its tail and launch itself onto structures. In such instances where a potential host is unable to move the Facehugger will slowly crawl out of its egg and onto the host. The resin deployed by full-grown Xenomorphs to cocoon a host also serves as a guide to show the Facehugger where its host is located. Once a Facehugger attaches itself to a host it cannot be removed or else the host will go into a state of shock resulting in instantaneous death. The Facehugger while attached will alter its host's DNA so that the suppressant used to render the host immobile will not kill the host. The two sacs on the Facehugger's body are how a host for the Xenomorph breathes while it is attached. A Facehugger can stay alive while submerged in water and still breathe for the host because the two sacs act as filters, pulling in needed molecules and letting out by-product molecules. Due to the epidermis of the Facehugger they can survive in extremely harsh conditions. The Xenomorph's ability to alter a host's DNA plays a key role in Embryo implantation, because the Facehugger is able to alter the DNA of the host so it will begin to grow the Embryo by itself. Once Embryo implantation is complete, the weak Facehugger will detach itself and crawl away until it dies. The blood is neutralized upon death rendering the Facehugger harmless. The life span of a Facehugger is unknown.

It's speculated the host's body begins to grow the Embryo due to the Xenomorph's altering of the host's DNA. The Embryo grows in the chest cavity just behind the host's sternum for protection, but unfortunately this means a very painful death for the host. Speculation also suggest the Embryo uses some of the host's ingested nutrients causing the host to feel hungry upon regaining consciousness. The Embryo introduces a chemical into the host's digestive system to keep other Xenomorphs from damaging the host. It's also known that a host will experience a slight fever after removal of a Facehugger as their immune systems reacts to the foreign tissue but due to the altering of the host's DNA the Embryo is able to develop unharmed. The time it takes for an Embryo to develop is seventeen to twenty-four hours, and with a Queen Embryo the time is more than twenty-four hours which is speculated to be because of the Queen's more complicated structure. On the U.S.M. Auriga the Embryos took only two to four hours to develop with a Queen Embryo taking only eight to twelve hours for development which was due to the mixture of human and Alien DNA. If a host dies with an Embryo inside him this doesn't necessarily mean the Embryo will die too because there's no evidence to suggest that it's tied into the host's main systems. The life span of an Embryo has yet to be determined.

The Chestburster punches through the host's sternum sending blood and fragments of bone everywhere killing its host. It's long and slender with small malproportioned limbs and a yet undeveloped head. The Chestburster moves by using its tail to push it along much like a serpent. One of the first things a Chestburster will do is make its first use of its lungs by letting out a scream or cry. After it does this it will leave its host's body. Strangely enough the Chestburster does not feed off the dead host's body, but why this is has yet to be determined. What the Chestburster eats while maturing is unknown, but it is known that the Xenomorph will molt its shell one to two times. Its metabolism is higher than any of its kin, and it takes it two to three hours to reach its full size of an estimated two meters. However, the amount of time it takes for it to mature is dependent on how long it took it to grow in the host. The life span of a Chestburster is unknown.

The Adult is two meters tall with a strong tail that extends to the height of the body, and it's generally bipedal. Speculation suggests that the Xenomorph is a polythermic creature due to the problems surrounding it being a endothermic or ectothermic creature mainly because it produces no body heat. This means it's the first of its kind. The long head of the Adult may be due to housing an enlarged brain which doesn't necessarily mean a greater intelligence but a more complex one. It has been speculated that a translucent cowl on an Xenomorph's head shows it's young, and as it gets older it loses the cowl exposing distinct textured lining like that of the Xenomorphs encountered on LV-426. The reason for the cowl is probably to serve as a protective shell for the still developing cranium. The Adult Xenomorph possesses the ability to climb on walls and ceilings which is thought to be due to suction-like structures on its hands and feet. The Adult Xenomorph has five spine-like appendages extending from its back, and the uppermost can be seen to be no more than an exaggerated vertebrae which could aid in warding off rear attacks or just to make the Xenomorph seem larger. The other four tube-like appendages have been hypothesized to contain glands that secret a type of resin much like that used to construct a hive which would aid the Xenomorph in dropping down onto prey or ascending great heights while leaving the arms, legs, and tail free for use. It's possible they are used to secret the resin that the hive is constructed of, but it would be easier just to have the Xenomorph regurgitate that substance. The Adult Alien encountered on Fiorina 161 did not have dorsal spines because they would've interfered with the Xenomorph's head when held in an upright position on all fours. So it adapted by shrinking the glands (normally present in the spine-like appendages) and moving them back to the anus. The secondary jaws of the Alien which are part of a modified tongue are used in an offensive manner and can spring twelve inches outward from the Xenomorph's mouth. The second set of jaws primary purpose is immobilizing a potential host but they can also be used to frighten hosts and activate buttons. The tongue's tensile strength is greater than bone, and once it reaches its maximum length its jaws close and its tongue is retracted. The Adult Xenomorph is able to spit a concentrated ray of venom toward a potential host that can dissolve flesh and enter their blood stream eventually rendering the potential host immobile. The Xenomorph's tail is very powerful and it's used for agility and balance, an offensive and defensive weapon, and immobilizing potential hosts with its stinger. In aquatic situations it allows the Adult Xenomorph to move with spectacular grace and agility. The Xenomorph's detection organs are all located in or around its head, and are extremely acute. It appears the Xenomorph feeds upon humans, but what its dietary needs are have yet to be determined. Also, what type of internal organs an Adult contains is unknown along with its life span.

The Queen is about four and one half meters tall with an extremely powerful tail that is equal in length to the Queen's height. It possesses a large cranial crown which extends two meters behind the back of its head, and it has a second set of smaller arms. The Queen's head is kept hidden in its cranium until awakened in which at that time it will emerge much like a turtle. The large crown may be used to make communication between it and the rest of its kin easier. The secondary jaws of the Queen are just like the Adult's secondary jaws except they're proportionately bigger with a striking length of thirty-six inches. The second set of smaller arms the Queen possesses was originally thought to be used in a mating process, but the Queen on the U.S.M. Auriga was born fertile, however, this may be due to the mixture of human and Alien DNA. A mature Queen has an ovipositor which can reach a length of eight meters. Both the Queen and its ovipositor are suspended from the roof of a hive by thick strands of resin. The total egg laying capacity of the Queen is speculated to be two hundred and forty. From the end of the ovipositor an egg is set down onto the floor of the hive. The Queen is responsible for making Queen-bearing Eggs but when it starts producing them is uncertain. Speculation suggests that when a Queen's Egg laying abilities are at an end the next Queen born takes over. Once the Queen encountered on the U.S.M. Auriga had laid its maximum number of eggs it lost its abdomen, and it was tied to the floor of the hive. It then developed a womb in which a Newborn grew. After the Newborn was fully developed it emerged from its womb and killed the Queen, but why as yet to be determined since she appeared to survive the birth process. The reason for the Queen on the U.S.M. Auriga doing this was due to the mixture of human and Alien DNA. The life span of a Queen is unknown.