|What are you? A man? Or a Titan?|
|— Kitz Weilman to Eren Yeager|
The Power of the Titans (巨人の力 Kyojin no Chikara?, also translated as Titan power(s)) allowed Subjects of Ymir to transform into intelligent Titans with specific characteristics and unique abilities, known as the Nine Titans. The holders of the Nine Titans were able to regenerate from injuries, and gained access to the memories of previous Subjects of Ymir that held the power before them. If a Pure Titan gained the power of the Titans, that Titan would return to being a human. After Ymir's decision to relinquish her power, all the Titans collectively lost their power and reverted to normal humans.
In order to transform into one of the Nine Titans, a human with the power of the Titans must suffer an injury that draws blood (often self-inflicted) while having a solid goal or belief in mind at the moment of injury. Being injured by another person is also sufficient. Once the transformation is initiated, a burst of energy is released, followed by the generation of the Titan body and a release of hot steam. After transforming, the human body rests within the nape of the Titan's neck as the center of control. The human body is also merged with the Titan's flesh inside to varying extents, most frequently on the upper area of the cheeks below the eyes. When a human with the power of the Titans transforms for the first time, they can often lose control of their Titan form and can become a danger to both enemies and allies.
If a Titan form is pushed to its limit, the human will emerge from the nape of its neck, sometimes in a semi-conscious or entirely unconscious state. If a Titan form is not exhausted, then its human possessor can emerge from the Titan form in full consciousness. In some cases, a human is able to partially emerge from their Titan body without fully leaving the nape.
After the Power of the Titans is used, the area around the emerging human's eyes is temporarily marked by dark lines. The design of the marks appear to vary on some cases depending on which Titan the human is in possession of, as shown by humans who hold the power of the Colossus Titan having more striking marks on their face than the others, resembling the Titan in their possession. The amount of time the human possesses these marks appears to depend on how long, skilled, or overworked they used their Titan forms, ranging from several minutes to several hours.
The ability to transform into a Titan can be hindered by injuries inflicted on the human body. A human can transform into a Titan with injuries as severe as a missing limb, but only if a previous transformation has not occurred recently. If a human emerges from their Titan form and receives a severe injury, this person cannot transform into a Titan again until said injuries have healed.
Those with the power of the Titans appear to be resistant to the normal absorption that creates mindless Pure Titans. However, this does not mean that they are immune to it. In the case of Eren Yeager, it has been shown that repeated transformations in a short period of time lead to decreased control over the Titan form, as well as diminished Titan size and power. As such, it seems that there is a limitation on how long a human can remain in a transformed Titan state before risking absorption. The length of time spent in a controllable transformed Titan depends on how skilled the human is, although even the most experienced can become incredibly fatigued for as long as several days after repetitive use. The exception to this appears to be the Cart Titan, which is known for its great endurance. Pieck is able to stay in her Titan form for at least two months.
A Titan's transformation usually causes some destruction to the area in which they have transformed in. However, the Colossus Titan is the only known Titan capable of increasing the blast radius to a larger scale than simply the area in which it is located, in which it usually resembles a nuclear explosion. This power was first seen being used by Bertolt Hoover during the battle of Shiganshina District, and later by Armin Arlert to wipe out most, if not all, of the naval forces during the Raid on Liberio. The respective blasts were seen by the full district of Shiganshina and the city of Liberio, and sent massive waves of energy inland.
Those with the power of the Titans can regenerate their bodies, as long as they have the will to live. They are even able to grow back their entire head in the event of a partial or complete decapitation. Regeneration from severe injuries such as lost limbs can strain their powers and prevent them from transforming for a period of time; healing takes much longer as a human than as a Titan. The time it takes to regenerate depends on the extent of the injury and how focused the individual is on healing that area. This healing is not absolute, however, and can be overwhelmed if the injury is severe enough. It is also possible to intentionally delay the healing, which Eren uses to infiltrate Marley as an injured soldier, or when Reiner was able to hold off on regenerating several wounds received during his time infiltrating Paradis. The only surefire way to kill a human with the power of the Titans is through severing the brain's connection to the rest of the body, although it is possible to survive even this by transferring the mind to the rest of the nervous system.
Due to the Paths, those who inherit the Nine Titans are able to see the memories of previous inheritors to some extent. Armin Arlert says that touching objects or people can help trigger these memories. Blood relations between inheritors play a significant role in passing these memories. Both the Reiss and Tybur families kept their Titans—the Founding Titan and the War Hammer Titan, respectively—within the family, and because of that, they were able to access memories going back by several generations, up to at least the Great Titan War. Other inheritors are shown to be able to see the memories of the previous inheritor at the very least. This process appears to fail when blood relations are interrupted or not present: Ymir and Armin did not receive any memories upon inheriting their Titans, in contrast to the Reiss family who seemingly gained their memories right away. Porco Galliard is unable to see the memories of his brother Marcel possibly due to Ymir being between them in the inheritance line, although Porco could see Ymir's memories.  Sometimes, the new inheritor will have dreams of a previous one, such as the case of Armin dreaming of Bertolt Hoover's final moments.
The Attack Titan is unique in that its inheritors can receive memories from the future as well as the past. The user of the Attack Titan is able to see events that their future self will experience, and even peer into a future inheritor's memories to some degree. Eren Kruger mentioned the names Armin and Mikasa to Grisha Yeager many years before the two were born, although he admitted that he did not recognize where those two names came from. Grisha would have some insight on the Fall of Wall Maria hours before its occurrence through Eren's memories. Eren had glimpsed future events multiple times throughout the years.
Like regular memory inheritance, bloodlines enhance the strength of the inheritance, but a person of royal blood can further enhance its abilities. Eren Yeager was able to glimpse a cataclysmic event as he kissed Historia Reiss' hand. Years later, he was able to enter his father's memories and influence Grisha; although he had to do this within the Paths and with Zeke's - who possesses royal blood - assistance. Using this opportunity, Eren was able to set many things into motion, the most notable being Grisha's theft of the Founding Titan. In these moments in time, Grisha was able to sense Eren and Zeke's presence, and was even able to hug and talk to Zeke.
While it has not been performed by every Titan that has been shown, it is noted in the series that all Titans can harden with enough practice and training. This hardened material is shown to be so strong that not even the Colossus Titan could kick through it, instead having to kick through a man-made gate. Hardened materials on Titans are shown to be penetrated by Thunder Spears, sheer strength (presumably by another Titan), or the power of the Founding Titan. Titan hardening performed around humans, has only been shown to be broken by the Jaw Titan's jaw or claws, along with the power of the Founding Titan, with things such as Thunder Spears merely bouncing off of the titan crystals. This skill has shown to be easier to learn based on the Titan, such as Eren's Attack Titan not having the skill to harden until a special serum allowed him to harden almost instantly after ingesting it.
Curse of Ymir
The first Titan, Ymir Fritz, was killed thirteen years after acquiring the power of the Titans. Because it is impossible for anyone to surpass the Founder, each person who gains the power of the Titans is fated with the "Curse of Ymir" (ユミルの呪い Yumiru no Noroi?), which limits their remaining lifespan to only 13 years after first acquiring it. During the last years of their life, their body will weaken until they perish. Eren Kruger, Uri Reiss, and Tom Ksaver were all sick and appeared to have aged considerably during their final year.
Royal blood allows the "full use of the power of the Titans." This is common knowledge in Marley. Only descendants of the royal family can fully utilize the Founding Titan's powers. Zeke Yeager, a royal on his mother's side, inherited the Beast Titan, but thanks to his royal blood, he can mimic some of the Founder's abilities, such as creating and controlling Pure Titans and even making them move at night using only moonlight; no other Beast Titan had these abilities. Unlike the Founding Titan, however, the control over Titans is not perfect, as one of them had shown some disobedience to Zeke's orders.
The power of the Titans can be transferred to a new host, should the current host be devoured by a Titan. This allows an ordinary Titan to become human again and possess the power of the Titans, and inherit one or more of the Nine Titans' abilities as well as the memories of the previous inheritors. This has been the case with the Founding Titan for over a thousand years.
However, according to Rod Reiss, the power of the Titans resides within a human's spinal fluid, meaning that to gain the Titan's power, it is not necessary to eat the entire human, but rather a Titan can simply bite through the spine and ingest said fluid. Titan serums are, in fact, the spinal fluid of Titans, though study of the serums is difficult due to the serum's rapid rate of evaporation upon contact with air.
If an individual with the power of the Titans dies before it can be transferred, the power will be passed on to a baby among the Subjects of Ymir, who is born after the said inheritor's death, regardless of distance or relation to the previous inheritor.
The source of the spinal fluid used to transform an individual into a Pure Titan can potentially affect the traits of their respective Titan form upon inheriting one of the Nine Titans. For example, the spinal fluid used to transform Falco Grice into a Titan came from Zeke Yeager, the Beast Titan. Because Falco's transformation involved spinal fluid from the inheritor of the Beast Titan, his Jaw Titan form took on characteristics of the Beast Titan, specifically avian traits. This process has evidently been used on the other Nine Titans throughout history such as Annie Leonhart's Female Titan, which she states had been subjected to extensive trait manipulation.
- Main article: Ackerman clan
The Ackerman clan, when awakened, can manifest the power of the Titans in some form without becoming or inheriting a Titan. They automatically gain the combat experience of every Ackerman before them via paths, similar to how memory inheritance works.
- In the August 2016 Issue of Bessatsu Shonen Magazine, Hajime Isayama was asked of what is it like to come out of a Titan's body. He stated, "Just think that it's like coming out from a sauna."
- In the November 2018 Issue of Bessatsu Shōnen Magazine, Isayama confirmed that Eren, Reiner, and others possessing the power of the Titans do not feel pain if their Titan bodies are damaged.