Conformity
The pressure to conform plays a major role in Uglies. The dystopic world in which the novel is set values conformity to such a high degree that it has instituted a compulsory cosmetic operation to ensure that all people look the same and are therefore equal in the eyes of others. Tally's pull toward conformity exists in opposition to her personal ethics, prompting the inner strife that provides much of the thematic tension in the novel. Her horror at the thought of staying ugly and becoming a social pariah is what drives Tally to work for Special Circumstances in the first place. However, when Tally learns of the brain lesions that result from the operation, her feelings change. While she wants desperately to fit in, Tally is not willing to sacrifice her individuality and her critical thinking skills.
Beauty
Initially, beauty is presented as a fixed quality in the world of Uglies, referring solely to physical appearance. Those who have undergone the operation are pretties, those who have not are uglies. Those who do not undergo the surgery have no shot at living a successful and happy life, because society will ostracize them. Consequently, uglies grow up with severely damaged self-esteem, unable to see their bodies in a positive light. But Tally's experience in the Smoke, and most especially her romance with David, complicate her understanding of beauty. She realizes that it is a relative and subjective term, applicable to a person's inner as well as their outer self. Accordingly, there are many ways to be beautiful, and diversity is what allows beauty to flourish.
Technology
Tally's world is held up, literally and figuratively, by modern technology. The bedrock of the society's structure depends on the social divides created by cosmetic surgery—uglies, new pretties, middle pretties, crumblies, and Specials. The complex architecture of the city is made possible only by "lifters," which function based on magnetic principles. Similarly, hoverboards are held aloft by the steel grid that has been installed underground. There are benefits to this advanced scientific technology: it has all but eradicated disease, greatly strengthened the economy, and eliminated poverty and war. But it also renders Tally's society more vulnerable to destruction, because its citizens are completely dependent on its continued success. Should their technology fail, the population would become utterly helpless, completely unable to care for themselves. At the beginning of the novel, Tally reflects on this thematic issue while surveying an old bridge spanning between Uglyville and New Pretty Town: "A million years from now, when the rest of the city had crumbled, the bridge would probably remain like a fossilized bone." Some technologies, like building with bricks and mortar, are more dependable than others.
Freedom
Tally lives in a surveillance state where her every move is witnessed and recorded by the government. She wears an interface ring that tracks her movements, and sleeps in a dorm with smart walls that listen to what she says. At 16, she must undergo the surgery that will turn her into a pretty, and prompt the growth of lesions that will result in intentional brain damage, so as to keep her docile. She has no right to privacy, and a limited ability to make her own choices. In the Rusty era, no governmental limitations were imposed on peoples' behavior, which led to total annihilation. Now, the government severely impinges on its citizens' personal freedoms.
When she journeys to the Smoke, Tally's eyes are opened to the pleasures of personal freedom. She becomes more confident and self-reliant. Her self-esteem, once in shambles, begins to rebuild. Though she must live without the protections and comforts of a centralized government, at least Tally has the right to privacy and the freedom to say no. Still, the question remains: to what extent should the government limit personal freedoms? What lines need to be drawn in order for a civilization to thrive?
Loyalty
The question of loyalty underpins much of Tally's behavior in Uglies. Her desire to remain loyal to Shay drives her initial resistance to betraying the Smoke. Once she arrives in the settlement, Tally's loyalty to her government is eroded by her love for the Smoke's rebellious, free-thinking inhabitants. The struggle to remain loyal to her government, her friends, and herself consumes Tally. To whom should she be allegiant? In the end, seeing the suffering her behavior has wrought, Tally understands the paramount importance of loyalty to oneself, not so much as it pertains to self-interest, but as it pertains to one's personal values.
Nature
The complicated power, savagery, and beauty of nature is another theme in Uglies. Tally, who has grown up in the city, has never been exposed to the terror and majesty of the natural world. She is entranced by the spectacle of the wilderness and terrified by its cruel indifference to humanity. Nature, she realizes, is on no one's side but its own. Often, Tally's emotional predicaments are illuminated through natural metaphor.
Body Modification
Everyone in Tally's world must undergo compulsory body modification surgery in order to be socially accepted. As a result, people are intensely self-critical, believing themselves to be unlovable and hideous without medical intervention. Though the novel does not take a strong stance on the issue of cosmetic surgery itself, it does advocate for increased self-acceptance.
Social Isolation
More than anything, Tally wants to belong. Her adolescence has been riddled with painful loneliness and longing; she believes that once she has the operation, she will finally feel as though she fits in somewhere. Fear of being a social pariah convinces Tally to work with Special Circumstances, a secret government agency whose methods go against her personal values. During her time in the Smoke, she pretends to hold the same rebellious beliefs as the other Smokies, so she will not stand out. Tally's dearest wish is to feel loved and accepted within a community, and consequently, by herself.